wrote:

Is 1000 feet the absolute minimum vertical separation between ifr
aircraft? Or can atc assign 3500 feet to one, and 4000 to another,
and have no lateral separation??

1000 feet up to FL290, then 2000 feet.


Secondly, any different for a situation where one is ifr at 4000 ft,
and a vfr aircraft is being vectored at 3500 feet, with no lateral
separation?

Depends on the airspace. If you need to have them separated then 500
feet will be the minimum.