Maybe 1 pr month... or every few months.

They could make two versions of PS+. One that gives you 3 PS4 games and one PSVR game, while one retains the current system.

Giving you 3 PS4 and one PSVR game might technically give you fewer games each month (if you own all three PS platforms), but you often get cross-buy dupes so you get one game on three platforms instead, so in reality you get 4 different either way games.

This way you could add value for those that only have a PS4/aren't interested in gaming on a PS3 or PSV anymore, and retain value for those that still play on their PS3/PSV.

Which months did that happen on PS+ that you didn't get 6 different titles? The cross-buy versions are counted as one of the six games. 

looking at this list (focussed on release/2016 titles) http://www.neogaf.com/forum/showthread.php?t=1044658 I think there should be enough to atleast throw in one of the smaller scale games for "free" each month

Oh. I guess that's what I get for talking about PS+ without having PS+

Still, reducing the net amount of games by two would allow them to focus more on providing a better lineup with the 4 games that remain, and getting one extra PS4 and PSVR game would increase the value of PS+ for those that don't have/game on a PS3 and/or PSV, and it would be up to each consumer which version they prefer.

Yeah I think so. I have a feeling we'll start getting 1 PSVR game in place of 1 of the Vita games.

Yeah. All the PSVR games are supposed to be playable on PS4 so it would still be giving PS4 owners more games.

https://blog.us.playstation.com/2017/08/30/ps-plus-free-games-for-september/

Here’s a bonus for PlayStation VR owners: PS Plus members in the US and Canada will also get RIGS Mechanized Combat League as a free bonus game from September 5 through November 7. RIGS is a first-person arena-based shooter developed from the ground up for PS VR. Set in the year 2065, it immerses players in an intense, all-action future sport that mixes elements from different athletic fields like combat sports, motorsports, basketball and football.

https://blog.eu.playstation.com/2017/08/30/your-playstation-plus-games-in-september-include-infamous-second-son-rigs-child-of-light/

As a special bonus this month, PS Plus subscribers will also be able to download PlayStation VR launch title RIGS.

Designed and built exclusively for PS VR, RIGS places you into the cockpit of a mechanical sporting titan known as a ‘RIG’ and launches you into a competitive sports league that combines first person shooter combat with team-based tactics and… erm slam-dunking yourself through a giant gold hoop.

It’s as chaotic and exhilarating as it sounds. Moreover, as you move through the league you’ll unlock new RIGs and upgrades, each tuned towards a specific role or playstyle that will broaden your options inside the arena.

It’s exactly the sort of gaming experience virtual reality was conceived to realise, where complete immersion and tactical awareness of your surrounding shapes how you play and your chance at winning.

I wanted to have RIGS so great.

If there are enough PS VR titles to support than, I don't see why not. I don't think there are enough though, although I could be wrong.