even if you do tell her how you feel.. it doesn't neccesarily mean that your friendship is over.. really it just depends on how strong your friendship is to begin with
if she really values you and the friendship you have to offer.. then she will at least hear you out and your friendship will more than likely remain intact but most importantly.. you won't have anything to hide
and as for being vulnerable.. how much do you trust this girl? would she willingly harm you either emotionally or physically?
just because you've developed feelings for her doesn't neccesarily mean its the end of the world.. but it does mean you'll have to tread softly for a while..
if you two really are as close as i'm understanding you are.. then if she doesn't have an idea how you feel already.. then she sure as hell knows your uneasy or holding something back (women are good picking up on these types of things) and that will put a strain on your friendship regardless of how strong it is
as was mentioned earlyer she just came out of a serious relationship (which is enough to rattle anybody's nerves) and the last thing she needs is added stress.. unfortunatly though it seems no matter which way you go theres going to be at least a little.. so it might be wise to kind of ease into things.. but definetly i think you should at least let her know how you feel.. and let her take the next step if she so chooses
but regardless of what you do.. a person is infinetly fortunate if they find even 1 true friend in life.. so take care not to be one of the unlucky ones
(please forgive my lack of punctuation i could never get into the habit of using it.. tends to interrupt my train of thought and i end up spouting BS.. which i may be doing anyway)