I think the obvious answer is to use floating point math ...
But then the frequency of your interrupts may become more important. How fast are they?

Or maybe you could take the remainder of the division and add it in a separate variable on each step, and when that overflows add one to Vel.
When ramping down, subtract 1 on an underflow instead.
It's kind of like floating point, as long as you're just adding and subtracting 1 unit each time.
It's harder to do than the actual floating point, but is probably faster.