I'm sunburnt and the pool party is tomorrow!!?

I went out today for lunch and was only out for a couple of hours but the backs of my shoulders got sunburnt. I have marks from the baby-doll dress I was wearing (straps and a line from where the top stopped just above where my shoulder blades start). It's already starting to hurt after just an hour and getting redder. It's pretty bad.

Tomorrow is Christmas and my fiance and I are throwing a pool party (we're in Australia btw, not as crazy since there's no snow). I'm not trying to impress anyone or look super sexy but this sunburn is gonna look really odd with my string bikini and make my pale skin really stand out. I don't have any bikinis that will adequately cover the sunburn and can't think of any that will work because of the back cut of the top I wore today.

I've already applied vaseline intensive care lotion in copious amounts (contains aloe vera and vitamin e) and intend to keep applying throughout the afternoon/evening. The pool has a giant shade/UV sail over it and I'll be sure to protect my skin from the sun's effects with SPF 50+.

Is there anything I can do to lessen the appearance of sunburn?

Will my color-stay/waterproof foundation do the trick or will it look odd?

Is there anything else I can use?