Some years back I was vegetarian for a year, and my bloodwork took a turn for the worse. I started working with a dietitian and got everything back on track in a couple of months. The most important takeaways, in my case, were to: get more protein; eat good fats/oils and avoid bad fats/oils; eat whole grains, avoid "white" foods; minimize sugar; avoid highly processed foods.
For what it's worth, I think that these are good general guidelines and can apply whether one eats meat or not. And, for the record, I reintroduced meat into my diet