How is this bronze rating awarded? What is 80+? Why does the VS series don't have that? ....well I got the answer already.
Say a power supply is claimed to be a 100(a clear round number for simplicity)W PSU and it actually supplies say 78W without stability issues, that PSU definitely won't get an 80plus rating...
...now a PSU that is claimed to be 100W and actually manages to supply 82W under constant load without stability issues, that would be able to get that rating.
So the idea is that as long as a PSU is capable of supplying 80+% of it's claimed maximum Wattage under constant load without any issues then it's applicable for the 80PLUS rating. As for the bronze, silver, gold, platinum etc. reference, that's just marketing jargon basically, though they do mean something, namely the more valuable the metal mentioned the higher a load percentage over 80 the particular PSU supposedly can manage.
So the way I understand it to be... lets assume that the VS550 is eh, best case scenario... capable of at most 80% load but not more, hence not 80PLUS, its combined 12V rail is claimed to be 504W so 80% would be ~403W I.E. you should only use it for a system that at most draws lets say 400W in total from the 12V rail.
edit: as for thread resurrection, not sure what the forum policy is in this regard however toms hardware threads do tend to come up alot in google search far as I can tell so it does seem worthwhile to answer such questions no matter how old.