Buying ps plus just for these platforms is optional so the only benefit is the free titles.  With that in mind how long will Sony keep supporting them?  Are there enough ps plus subscribers that only play on vita or ps3 to justify support for many more years? 

As long as they keep their online services open, why would they stop getting games? PS* users already pay a ridiculous fee for that service, I'd be angry if my old systems stopped getting games.

This is something I've thought about a fair amount. I hope they continue supporting both with Plus games for as long as possible, but the day is coming....I'm wondering how long the PS3 and Vita will have games available in the PS store in general.

I'd expect until at least 2019. Doubt it costs them much and adds value to PS+ for them to market.

Practically never I would assume.

Someone who's actively playing PS3 / Vita games may not necessarily have a PS4. PS+ in this case helps retain that person as a customer

As someone who hasn't downloaded a Vita or PS3 plus game in years, hopefully sooner than later. I'd rather see that money put towards more PS4 games or more AAA PS4 games or PSVR. I'm sure they have internal data showing a enough people still download them to keep releasing them.

Before the end of 2018 for PS3, maybe Vita will last a year more.

What money? The money it costs to keep ROMs on a server? That's pennies on a dollar compared to AAA game development. Sony is making far more money keeping millions of PS+ subscribers and gamers in its ecosystem than it is hoping to sell a few million copies of games that cost hundreds of millions in development, publishing and advertising. If you want Sony to keep bankrolling more games, you should want this service to stay available for as many subscribers as possible.

According to this post, the developers of the PS+ games don't get a cent from the games being downloaded via PS+. Sony pays the developers to put their games on PS+. The fewer games available on the IGC, the more the developers would get paid and the better the IGC will be.

Personally, I think 2019 would be the cut-off year for the PS Vita and maybe a year after for the PS3. I think the number of people playing on those systems will not be massively significant by then.

^According to that post up there^, Sony spends buttloads of money for PS+ games that rival the cost of AAA game development, which has the unfortunate side effect of limiting the amount of PS4 games it can provide consumers. Sony isn't making money hand over foot with PS+, and keeping the service alive for PS3 and Vita is a humoungous albatross around its neck, but the world's leading scientific minds can't quite figure out why Sony doesn't just end it for those old consoles. BREAKING NEWS!