Yeah prior to W10 memory could be allocated even if not in use, beginning with W10 (I think it's 10, u can look it up) any VRAM not actively in use with data residing in it must be given up so other apps can use it, = Memory Reclaim.I don't have GTA V so I can't speak for experience but, if my memory serves me correctly, that game uses extra VRAM for caching so it could very well be just like TW3 in that respect, so I wouldn't go making any assumptions to that end.
I've played TW3 maxed out without Hairworks. In fact I didn't find Hairworks to impact performance by all that much, I feel that it's just a slow engine for what it is doing. I don't have GTA V so I can't talking about it but, there was a discussion several months ago about how it appeared that GTA V was caching stuff in VRAM if it was available and wasn't reflective of how much memory is being actively used at any given time.
More of a Windows Memory/Driver Management problem than application afaik..
Affects games DX 11< onward.