I don't know anything about Arturia sale. I'm a user.
But If V-collection 3 (VC3) is on your account, then i can't see why you should not get the discount as a VC3 owner.
I can't see why it should depend on it's activated or not. Don't you have an offer from Arturia on your account for VC5?
Otherwise you need to contact Arturia sales about this.
You can have more V-collections activated at the same time. But i don't think you need to activate earlier V-collections to activate a new VC5 offer/ upgrade.
If you have downloaded and installed the latest versions of the VC3 collection from the download page, then the activation thru ASC should work.
You have 5 activations. So you can install V-collection on 5 computers.