Whenever a big, relationship-breaking problem rears its ugly head, you have to go through a process together to see if you can work things out or if it's not going to be possible to stay together. While you're going through that process, it can be helpful for both partners to try to find some understanding for the other.
My post is, therefore, about ways that you, Podunk, can find some understanding for Veganchick. I don't know if my ideas will be helpful or relevant or not. I don't know everything about the situation. But here are some things to just to think, just to mull over.
I don't know Veganchick's side of the story because she hasn't written it. So I can't give her advice, and I'm not even going to chastise her for her behaviour. This post is for you, Podunk, because you can't change what she did (not even she can change that now, actually) and you can't change how she's behaving, but you can change how you're reacting if you want.
Something to understand I:
It can be hard to tell your partner about your sexual adventures, and it looks like it was hard for Veganchick, since she let you know in little trickles.
You know, both me and my husband have done a similar thing. Before we were open, officially, I cheated on him with another guy. I was pretty sure it would be okay with my husband. We'd talked about being open before. But we hadn't made an official agreement about it yet, and I knew I was supposed to ask first before I went and had sex with someone else, but... I didn't. Once I did the deed, it was done and I couldn't take it back. But what if I'd just ruined my relationship? Oh god.
So I didn't tell him for 2 months, and I was so miserable in that time, dropping little hints and worrying about hurting him. When I finally told him, my husband said, "Oh, you only cheated? Thank goodness! I thought it was something serious, from the way you were acting!"
We officially opened up our relationship, but even then, many months later, when my husband first slept with someone else, he didn't tell me right away, either, even though he knew he had full permission. He came home the next morning after staying overnight at a friend's place, and said that nothing had happened. Later on that day, he managed to blurt it out. I asked, "Did you not tell me earlier because you were afraid to?" He nodded, and I gave him a hug. "I know what that feels like," I said.
Veganchick was obviously afraid of your reaction to her news, because she couldn't bring herself to tell you the truth. A lot of that is her issue, but we're not focusing on her right now, we're focusing on you. Are there things about the ways you relate to each other that would make it extra hard for her to tell? Or can you perhaps forgive her for being afraid, if she can find it in herself to come clean now?
Something to understand II:
She may have been afraid of how you'd react if she told the truth. I'm not saying that excuses her lying, but it's Something To Understand.
You imply that you'd have been mainly fine with her actions if only she'd told them to you straight-up, but I don't think that's true. There are some things you've written that... give me pause. Some things that sort of smell very faintly of slut-shaming.
I didn't hear much from veganchick these first few days, guess it's hard to talk with a dick in your mouth?
Woah, cheap shot alert! Do you exude that attitude when you talk to her usually, buddy? Take a deep look inside and ask yourself if you're the easiest person to be honest with.
I also hear just a little patronizing language in how you talk about Veganchick. You constantly refer to her "youthfulness" and portray her as a naive. You cast yourself as someone who can guide her into her true potential or maturity. If there's a little bit of that dynamic in your relationship, that could also be making it difficult for her to "confess" to you, because she doesn't quite feel equal.
Something to understand III:
Reconsider your rule about unprotected oral sex. The studies are pretty good that it's low-risk behaviour.
We have to think carefully about the risks of having several sexual partners, but we all do take some risks. Would you require her to use a dental dam to kiss another guy, for instance? No, probably not... but some people have herpes simplex on their mouths, which can technically shed asymptomatically. Yet kissing is often a risk we take, because it's such a small risk.
I'm glad she's having an STI screen now, but that shouldn't be something that she does just if she "cheats". She--and you--should have updated STI screens done every 6 months or so if you're planning to continue sleeping around. She could also insist that her outside partners do the same. Perhaps that might be an easier rule to enforce than "no blow jobs without a condom"? Just a thought. Obviously, if you're absolutely not comfortable with that, disregard this.
Okay, those are some things to think about.
Also, moving forward: maybe it might take some pressure off if you took a break from negotiating this for a day or two? Say, "Look, Veganchick, you haven't been honest with me about what happened while you were away. I don't know if it's because you were afraid to tell me or what, but it's hurt me a lot. But we've been over and over it and we aren't getting anywhere. Do you want to, like... get a babysitter and then go to a movie and take a break from this for a few hours? I really want to talk about this more, but let's just relax for a minute."