There's several variations on this (besides うぃーす, there's also うっす、おっす、ちーす and probably a few more that I'm not aware of), all of which are very informal masculine greetings. They don't really have a good English analogue - something like 'hey' or 'yo' or 'what's up' is probably the closest you'll get.
These are fairly masculine greetings, and the average girl probably won't come across much of a need for any of them - if I heard a girl use one, I'd assume she was just being silly. These are also very informal - you'd only ever use them with your equals or inferiors, and mostly only in informal situations (i.e. AFAIK two teachers might greet each other at a bar with one, but wouldn't greet each other at school with one). You could probably get away with calling them slang.
It's not correct to write it as うぃす, as that doesn't accurately reflect the pronunciation. As far as I know, no one shortens that vowel, so うぃす doesn't actually mean anything.