There is nothing to fix.
Different games need different amounts of VRAM, they won't use any more than what they need.
ACS just needs less VRAM than some of your other titles, so that's all it will use.
"in assassin's creed syndicate settings it detects only 1990mb"
Wrong - - it's using 1990MB of the 2000MB that's available.
Like I said, there's nothing to fix.