Ashton Kutcher's alleged mistress -- the FIRST alleged mistress -- claims she feels "vindicated" by Demi Moore's divorce statement ... telling TMZ she hopes people will finally believe her story about sleeping with Ashton.

TMZ spoke with Brittney Jones -- who claimed she got busy with Kutcher in 2010 after meeting the guy at a bowling alley. Jones' credibility was called into question when she capitalized on her sudden fame by releasing a sex tape .... which did NOT feature Ashton.

Ashton's lawyer released a statement at the time Brittney was in the headlines, insinuating her story was a lie ... but not entirely denying

Now, Jones tells TMZ ... "Although divorce is often sad I do feel somewhat vindicated."

She adds, "For so long people have thought that I was dishonest or just making up my passionate nights with Ashton, when in fact I was being used."

"Ashton told me that both he and Demi had an 'open relationship' and that he was not in fact cheating. Now I can tell all the facts about how Ashton really was, and hopefully people will believe my side of the story."