Given that he was playing recently and regularly, and in view of Paul Warne’s comments, this was pretty clearly a Vassel rather than club initiated move.
You can tell from his body language when he is substituted that he feels strongly that he should be playing the full 90 minutes every game - and is pushing to get that at another club by putting his hand up to be loaned out, There can’t be many fans who would agree with his sense of entitlement to more playing time in a Millers shirt so his departure is probably a good thing.
He has a lot of energy and at times looked good in League One but he really isn’t much of a footballer and, more importantly, much of a goal scorer - or maker - in the Championship. The opportunity to replace him with someone better has to be a positive. Let’s hope it happens...