I have to convert a few ASP pages (to ASP.net) that set values for hidden input values from a database and does a POST to an ASP page hosted on a vendors website.
I already have found that ASP.net pages can post to webpage other than themselves... My question is in regards to compatibility problems with the sending end being ASP.net and the recieving end being classic ASP. Is it possible to post from an ASP.net page to a classic ASP page that's on the vendors site?
Yes, it's quite possible.
POST is a request mechanism defined by the HTTP protocol - it's technology agnostic. A web page at the end of the day is just a bunch of HTML and script, regardless of the technology that rendered it.
You may get some extra data that ASP.Net puts into your forms, but otherwise there's no reason you can't post to any web address you want. Note that you won't be sharing session state or application variables, and you may have to manually construct a form or write your own form control (the regular pattern of having one form tag with "runat=server" won't help, because this sets it up to do a postback).