Many ps1 games for vita are disappearing though.

You should check ps store on vita to see how many games are still on there.

Really? 

https://www.resetera.com/threads/how-powerful-is-the-vita-really.58893/page-2

PC with GTX 1080 [8TFLOP 2016 Pascal architecture, capable of mixed precision*]
Xbox One X [6TFLOPs AMD custom 2017 architecture]
PS4 Pro [4.2TFLOPs AMD Vega 2016 Architecture, capable of mixed precision*]
PS4 [1.84TFLOPs, 2011 AMD architecture]
XBO [1.33TFLOPs, 2011 AMD architecture]
Switch Docked [393GFLOPs, 2015 Maxwell architecture, capable of mixed precision*]
Switch Handheld [196GFLOPs, 2015 Maxwell architecture, capable of mixed precision*]
Wii U [2009 ATI architecture, 176GFLOPs]
Playstation 3 / Xbox 360 [2005 architecture, ~250GFLOPs]
Playstation Vita [2011 architecture, ~35GFLOPs]
XBOX [2001 Nvidia architecture, 20GFLOPs]
Wii [2001 ArtX architecture, 12GFLOPs]
Gamecube [2001 ArtX architecture, 8GFLOPs]
3DS [2009 architecture, 6.4GFLOPs] (pica200 @200mhz is 4.8GFLOPs, 3DS runs the gpu at 268MHz, so 6.43GFLOPs)
PS2 [Sony 2000 architecture, 6GFLOPs]
PSP [3.2GFLOPs, 2005 architecture]
Dreamcast [1.4GFLOPs, 1998 architecture]
N64 [190MOPs (millions of operations per second, over 100MFLOPs available)]
Saturn [110MOPs] (likely around half for MFLOPs)
PSX [66MOPs] (likely around half for MFLOPs)

ARM11 CPU: 2x MPCore/2x VFPv2 Co-Processor at 268MHz. Doubling to 4x MPCore/4x VFPv2 Co-Processor for new 3DS with 804MHz max clock.
ARM9 CPU: ARM946 at 134MHz.
GPU: DMP PICA at 268MHz (6.4 GFLOPS, 20.5 Million polygons per second at 268MHz, calcutated from 15.3 Million figure and 4.8 GFLOPS at 200MHz)
VRAM: 6MB, 10MB for new 3DS.
DSP: CEVA TeakLite at 134Mhz. ...
System memory: 128MB, 256MB for New 3DS.

3DS by far. At the time I was fine with my Vita - amazing hardware (original model is built like a tank) and some nice exclusives like Gravity Rush, Danganronpa and Tearaway. But then they got ported. There are better versions of those games now.

I never liked the Dual Screen nature of the DS and the 3D part of the 3DS. After spending _years_ trying to find games, beyond the niche stuff like brain age, to enjoy on the DS, I had already given up by the time 3DS had come around. Keep in mind that I was still using my GBA weekly. Vita on the other hand being a 'traditional' handheld faired quite a bit better for me, despite its clear lack of attention from SONY many times throughout its life. However, Vita made it clear that I was no longer enjoying playing games on the go or in any place other than my home in front of my big TV, and ended up being being put away in it's box permanently only a few years after I bought it.

So I picked Vita, even though I have to acknowledge that I don't enjoy handheld/mobile gaming anymore.

Yeah, also wasn't a fan of the two-screen-concept.

Most developers didn't know what to do with the second screen.

On DS most developers focused on the lower screen and its touch/stylus input, the upper screen was often "wasted" for ambience stuff.

On 3DS that shifted: most of the action of the games was shown on the bigger upper screen and the lower screen was reduced to a fancy touchpad.

Only a few developers managed a good balance between both screens.

And then came the Wii U with some games that constantly nagged with texts like "now look at the TV" -> "now look at the controller display" -> "now look at the TV"...

3DS CPU is a 2002 chip

That's a shared flaw of the Vita and Wii U. Some of their big games (first and third party) got ported.

Wii U: Most major first-party titles got ported to Switch or 3DS. Also Zombie U, Lego City Undercover and some other games went to other platforms.

Vita: Gravity Rush and Wipeout 2048 got PS4 Remasters. Tearaway got an expanded remake on PS4. Persona 4 Golden got a PC release in 2020.

Whether you got Vita when it was newer or you're getting it now, you're not really getting it for games exclusive to it. You're getting it for portability, remote play, PS1 and PSP digital games, certain shared games, etc. 

The 3DS on the other hand has tons of games that you can't get anywhere else. That helps me pick the 3DS in addition to having far more time with it than Vita.

I don't see it as a flaw when good games get ported so more people can enjoy them. Fuck exclusives.

Some good games of the 3DS also got ported, and that is also great:

• Resident Evil Revelations got ported to PlayStation + Xbox + PC + Switch
• Shovel Knight got ported to PlayStation + Xbox + Switch
• Steamworld Heist got ported to PlayStation + PC + Switch
• Steamworld Heist got ported to PlayStation + PC + Switch
• Retro City Rampage got ported to PlayStation + Xbox + PC + Switch
• Ace Attorney: Dual Destinies got ported to iOS + Android
• Ace Attorney: Spirit of Justice got ported to iOS + Android

And I would love to see many more 3DS ports (and DS ports) on Switch adapted to singe screen.

While I agree with your points I just want to point out that the PS4 version of Tearaway is worse than the Vita version. It's one of these cases where bigger =/= better.

3DS and it's not even close. I do like the Vita, it's a cool handheld, has some interesting things going for it with the back touch panel (which I really wish somehow made it's way over to other platforms later on because it's so cool) and the power for it's time. I also do think it had some great games. But the 3DS is legitimately my favorite game console of all time, bar none, between all home consoles and handhelds, all companies. It has an insane catalog, to the point where I have well over 100 games for it and I'm still finding new experiences. Even now in 2021, I continue to use my 3DS very frequently, it's almost always on, and I still bring it with me when I leave. (which these days is mostly just for work, but still) It has so many hidden/underrated gems, and many of them are first party titles that for some reason never caught on or got negative press for one reason or another. Code Name S.T.E.A.M. is one of my all time favorite games, Dillon's Rolling Western (and especially it's sequels) is the best tower defense game ever made, and clearly came at a time when Nintendo was willing to put out a $10 eShop game with 25+ hours of content. Not to mention some other underrated games like Ever Oasis. And on top of that you have the massive library of games that actually WERE well appreciated. Plus StreetPass, and the 3D effect which I actually really enjoy using for a lot of games. The best 2D Zelda game, the best Kirby game, Kid Icarus: Uprising, and many more.

I'm starting to ramble because I really could go on and on about the 3DS, I seriously love it so much. I wouldn't be surprised if I love it more than almost anyone, and I don't blame anyone for not choosing it, but maaaan I could play 3DS games endlessly.

There's quite a few games that make me feel this way. One of the obvious examples is FF7R, I beat it recently. I think I prefer the original in every way except the visuals obviously

Sometimes something short and sweet is better than something that's overly long and drawn out.