What is "anticipated separation" in this context? For example, in the airport environment "anticipated separation" specifically means applying either 3-9-5 or 3-10-6.
5-6-1c says you can vector "at or above the MVA" which means that as long as the aircraft is always at or above the current MVA then vectoring is legal regardless of their eventual/assigned altitude. But are you allowed to assign that lower altitude in the first place? The main place where minimum altitudes are discussed is 4-5-6. You must assign an altitude that is at least the MEA, except that in certain circumstances you can go lower. One exception is 4-5-6a2(a) which says that when radar procedures are used you are allowed to assign altitudes "at or above the MVA along the route of flight." I guess it's up to personal/professional judgement what "along the route of flight" means.
In the final analysis I think this is legal provided the aircraft was continuously at-or-above the MVA for the sector they were in, regardless of assigned altitude. It would be way too inefficient otherwise. But as 32 said, it's not positive control. Then again, when you're anticipating runway separation that isn't necessarily positive control either.
Two addendums: to head off any argument that 4-5-6 doesn't apply because the aircraft is on a random route and there isn't any MEA in the first place, I will ask, then what other rule says you need to adhere to the MVA? I don't believe there is another rule, except 5-6-1c, and for the sake of argument let's assume the aircraft is cleared direct to a fix rather than on a vector. So 4-5-6 is what we've got, unless you want to argue that you can clear them straight to the ground and I don't think anyone is arguing that. And the other radar rule you can apply is 5-5-9b which says that if the MVA sector is to protect for a specific and displayed obstacle then you can drop the aircraft to the surrounding MVA once they're past the obstacle itself, but again for the sake of argument we'll assume that doesn't apply to your situation.