This is the FAA's certification requirements for Vmc:
1) Aft CG (at rear CG limit)
2) Trim in takeoff position
3) Out of ground effect
4) Maximum takeoff weight at sea level (or at a weight that produces most unfavorable Vmca)
5) Maximum available power or takeoff power on operating engine
6) Flaps (wing) in takeoff position
7) Flaps (cowl) in takeoff position
8) Landing gear retracted
9) Standard atmospheric conditions (29.92”, 15 deg. C, sea level)
10) Bank up to 5 degrees into operating engine
11) Prop windmilling (or feathered if equipped with autofeather)
12) No more than 150 lbs. of rudder pressure required
13) Must be able to maintain heading within 20 degrees
Flaps up and Vmc will increase
Gear down and Vmc will decrease
Fwd CofG and it will decrease
Cowl flaps closed and it will increase
> standard temp and it decreases
No bank and it increases
In ground effect and it decreases
etc.....
Regarding the Seneca II, this actually generates most power at 12,000'. Rated per side to 200HP at sea level, 215 Hp at 12,000 (which is why in FAA land you need a High Performance endorsement to fly one
)