I believe that veto can be used sanely and responsibly to keep toxic and destructive people out of your relationship when your partner may be a bit too wonked out on NRE to see things clearly. The OP's situation does not appear to be this, of course.
In my primary relationship, we have decided that veto is permissible if a third party has substance problems that are creating issues in the relationship, if they do anything to endanger our children, or if they fail to disclose things that we need to know for our sexual health. Mental instability may be grounds for veto if it creates significant issues in the relationship. Lying may be as well, it depends on the seriousness of the deceit. We had a situation some years back where my husband's brand-new love interest was telling me, unsolicited, how great her marriage was, while she was telling my husband she was unhappy and wanted out. (No clue why it never occurred to her that he and I actually talked to each other now and then. That's not the only reason I vetoed her, but it was a contributing factor.) Trashtalking the primary, and then continuing to do so after a warning, is definitely grounds for veto with us, we don't insist that our partners become friends but we do insist on mutual respect.
I have been on the bad end of a veto myself, similar to the OP's, his wife threw a hissy fit that lasted for months, because we fell in love. She never owned what was really bothering her, but it was obvious. They as a couple never owned that a veto was what was happening, but that was also obvious, since he and I were having a successful long-term relationship before she threw her tantrum, in fact, we had just fluid bonded, with her consent.
I am so sorry that this happened to you, AlwaysGrowing. I know how painful it can be.