The trouble is that any version of America First that doesn't see that there are outcomes to foreign conflicts that are not in America's interest (e.g. Putin being able to reassemble the Soviet Union or Russian Empire, take your pick) that are can be prevented by America taking a side in the conflict, is that will not place America first in anything other than the estimation adherents of that misbegotten conception of national interest.
If we can get rid of the not-invented-here expression of excessive American self-regard that naive "America First" policies of Vance's sort also express, we could even steal a march on the Chinese by supporting Ukraine, going into joint production of their battle-tested UAV, UGV and UNV technologies, and copying their military development and procurement cycle.