I found that acetone works pretty well for stripping off the Vallejo primer.  I made the mistake of pre-priming my Harrier model before its assembly with Vallejo Surface Primer.  After fitting, sanding, gap-filling, etc., I found that the primer (even after curing for weeks) would lift and peel, instead of sanding smooth and flush.  To take it off, acetone/nail polish remover on a cotton bud worked pretty well (it's a 1/72 scale).