I used to ski Jackson Hole every year, and usually in the Spring. April seems later than I remember, but definately in late March.
I found that if the hill is high enough, it is still quite cold at the top 1/3 of the mountain. Also skiing in the morning helps. Get off the slush by 2pm.
The first year I went to Jackson, I made the mistake of going to the top of Rendezvous Mt (highest point) first thing in the morning. The snow and ice up there was bullet proof, and it was way to cold. Was great later in the morning.
Never been to Vail, so don't know much about the spring conditions there. But hope this helps.