If you got 5% per buff level, level 4 would restore 20% - 20% of, say, a 1k crit (not a rarity) would be 200 - you'd never see someone's HP further than one hit below full again.
We tested it with VR 3 and it was approximately 15% with no otehr vamp effects.
Was this when hitting you? You might be off by a factor of 2 because of transfer pain -- when they hit you they should get 8% back of the total damage they dealt (to you and also to your summon). If you don't have TP on, the number should be 8% exactly.