Vlaar says Villa were "deflated" by the quick succession of goals at St Mary's and couldn't get going as they strived to get back into the game following the second half flurry.
He told Villa's website: "We played fine earlier in the game. If you looked at the first half and then just after half-time when we created two good chances to score a second goal, right after half-time, we were doing well.
"We scored a goal, we looked quite comfortable in the game and, as I said, we might have gone two-up had we been able to take those chances early in the second half.
"Then very quickly and out of nothing, they scored an equalising goal and close to this they scored a second goal and then a third goal and that deflated us. We didn't play well enough after this to get back into the game.
"It was almost like we couldn't get going again. That's just not good because we played well in our previous two games and this gave us confidence."