I presume blitz is removing the texture from vram for object which are not visible.
No it isn't. 3D and texture management is handled by DirectX, Blitz just sends info to it what it would like to have on screen and DX does it (broadly so---). Usually you DON'T want to mess with VRAM flags unless you really know what you are doing and why...in this case I'd leave it untouched.
Lag can also be a result of many things, like camera placement (too close-> fillrate might get a hit).
You're loading 8 2048*2048 textures if I guessed right - that's a lot of pixels so unless you have a very speedy card and 256-512 MB VRAM on it you will get swapping - and that might slow it down occasionally. Fast camera paths / movement can also cause lag.