Disclosure of vulns and its legal aspects...

Hi all,

It was earlier today whilst testing some websites as a personal
research/leisure time that i found a quite critical bug in a major
computer related website which will not be disclosured until all the
legal aspects of the disclosure process itself are dealt with.
After detecting the aforementioned vulnerability i was, like many have
been before, "jailed" between the decisions of reporting it or not, it
didn't take me long to decide to report it to the vendor as the flaw
itself was on it's website... My first step and only one so far was to
write the vendor the typical "praxis" e-mail saying that there MIGHT
be a vulnerability SOMEWHERE on their website and that i would like
carte blanche to investigate a bit more about it. I am now stuck with
3 thoughts, first of all, if the answer is no ( most common perhaps)
the vendor will be losing its chance to know where and what flaw is
it... will i be stuck with that and not be able to publicize it to the
security community?
Second thought, if the vendor says yes, i will report them the
vulnerability but, what entitles me the right to do it legally... a
simple e-mail would be enough perhaps...
Third and last thought, if they indeed agree to give me the chance to
test and report them the vulnerability i will only be entitled to
publicize it once solved, but even then, will it be legal to make a
full disclosure?

Thank you all in advance,

Darkcoldice

PS: What would the difference be between the US and UK laws on that
final aspect?