As said by Martell - There's no way to 'add' vram. You have to buy a new card that has more. However, even on triple monitor setups 2gb can work fine. Decrease (or turn off) anti-aliasing (it uses a lot of vram). Decreasing other graphics options (like texture resolution/detail) can help reduce vram usage as well. But if you aren't willing to decrease graphics settings, and you're vram bottlenecked, you'll have to get a different graphics card or cards.