I just looked up this thread, since I wanted to try this again (last time I didn't push through), and saw this comment, and I wanted to say that I used to have that exact same experience with my heart function in relation to food. That's also why I thought for a long while a vegan diet would not be possible for me, since my heart would only function in a normal fashion as long as I ate full meals of meat and vegetables. I didn't know about the relation to vitamin B, though, I just knew it was linked to my diet. I just knew that when a friend came over and we only ate pizza and chips, and drank a beer, my heart would keep me up most of the night.
However, over the years I came to realize it wasn't just what I ate, and it wasn't just meat. For example, it's also related to physical exercise, vegetables and fruits. For example, I'm right now eating extremely low amounts of meat (after reading Foer's Eating Animals, I want to minimize it as much as possible) and I've noticed it's not a problem at all at the moment. This is because I exercise regularly (I run at least twice a week), and every evening, I have a salad of greens. That's the most important part; green vegetables, and fruits with potassium, like bananas and citrus fruits.
I also notice that when I reduce my running, greens and fruits, the problem returns, so as long as I'm on that schedule, I can afford to reduce meat and dairy without my heart protesting.
Have you tried Alpro Soya's plant based alternative to yoghurt? I switched over to it the last few weeks, and I'm not missing yoghurt at all any more! They have an unsweetened version with no carbs, no sugar, that is completely fulfilling my yoghurt cravings in the evening.
https://www.alpro.com/uk/products?products=plant-based-yogurt-variation
For me, it's especially the combination of a well filled salad meal every evening, often with some nuts, and an hour before bed I take this plain, unsweetened yoghurt alternative with Cruesly. Combined with regular exercise, I haven't had heart problems in months.