Having skied the last 6 years at Easter, it is generally pretty warm. I tend to keep one layer and a jacket on though, as it can get cooler on the lifts and I'm a wimp. Last year it was a full on blizzard as we left on the 7th so don't assume the weather will be amazing every day, but I doubt very much you get -15 or anything!
Val thorens is high and on the glaciers, well above the melt line you'll find it can be cooler. I would go with 1 more layer than you think and take it off and put it in a bag if you are hot at the top. I'm a cold soul.
I would aim to get high every day, get high, stay high and then go to a different area the next day. I hate the slush though so tend not to want to descend after lunch unless it's the last run of the day.
Mornings can be very icey. Make sure your skis are very sharp and it makes a world of difference.
However the warmth is lovely. We skied at Xmas this year and -30 was unbelievably cold. Our daughters instructor has frost nip on her nose. The ability to sit outside in the sunshine is a great way to finish the day. I think the snow conditions are a bit of a compromise, but we've always managed to find good powder and have been amazingly lucky with the weather really.
We were going to head to VT next Easter (change of plans now) and I think it's one of the top Easter destinations because of its height.