I was just waiting to get Blood Rush, trying to do bites with my other skins in the forums while waiting, and then decided to drink a Stinging Vial for the achievement. Now I'm in Stage 1 of the Stinging Vial, and not showing Blood Rush symptoms. It said drinking it "will not remove your morostide or other zombie effects", but does that count for infections I'm trying to start? Have I made a terrible mistake?

I actually do think that the vial negates any other virus you're attempting to get. :/ I can't 100% confirm this however, so don't lose all hope.

I'm trying to use the new cancellation option, but it doesn't look like it's available. Nooooooooo

The Vials aren't exactly a "virus" or a staged unlockable skin like the ones available on the VIP, so the cancel option won't work on them. You can't do anything other than wait for it to reach the first stage again.