Yes they can - a CDD (customer due diligence) or EDD (enhanced) trigger can occur on a sufficiently large withdrawal. The UKGC rules say that "players must not be asked for information at the point they request a withdrawal from their account if the operator could reasonably have asked for this information at an earlier time."
So delaying a CDD request until the withdrawal (as some operators appear to be doing, but UKGC sleepy) isn't allowed. But it is possible that the CDD is triggered by the withdrawal - where the rules are slightly different.
On the second part, yes - once the CDD request is active then the balance is ringfenced, subsequent deposits are added to it but importantly subsequent winnings are isolated. So if the CDD is completed then the balance + subsequent deposits + subsequent winnings are returned to the player, if the CDD is refused or otherwise cannot be completed then only balance + subsequent deposits may be returned and subsequent winnings may be lost - which is why operators delaying the CDD request is problematic because it gives them a freeroll against the player.
So in this case, if the withdrawal is the CDD trigger, then the balance should include all of the winnings to date.
VS have done things over the years we may not agree with, but this one comes from the UKGC and AML regulations...