Protein shakes are pretty straightforward, even for diabetics! The goal is to get a good amount of protein immediately after you workout, and there are several options: whey (if you consume milk products); pea protein (if you are vegan); and some others you can find in specialty shops.
You can make sure you get protein with no carbs (Isopure whey is what I use), but beware adding a lot of fat to a post-workout protein shake. It dramatically slows down the uptake of the protein, and either straight protein or protein plus a strategic amount of carbs is generally considered to be more effective (for diabetics and non-diabetics alike).
If you are lifting heavy (even relatively heavy, so that your muscles are getting good soreness after a workout), you probably need a lot more protein in your diet than most people (especially vegetarians) are used to eating. A rule of thumb for beginning weightlifters is 0.8g of protein per lb of lean mass per day, or 1.0g of protein per lb of body weight per day. That's a lot of protein, for most people, and you have to really be working hard to utilize it, but you won't be able to add (muscle) weight if you aren't doing both.
I'm sort of in the same boat as you: trying hard to maintain/gain muscle mass, which I've lost a ton of since diagnosis. I've recently taken to upping my carb intake (approximately double what I was a few months back), and it has stopped the weight loss for me. My blood glucose is not as tight as I'd like it to be, but I have stopped losing weight. I may have to come to terms with being one of those rare people that low-carb high fat diet doesn't work for: my body seems incapable of efficiently digesting fats, so even when eating 3,500 calories per day I was losing weight. By adding some carbs back (I'm eating about 100g per day, now), I've been able to maintain weight on a 2,500 calories per day diet. Long and short of that story is pretty simple: every person's metabolism varies significantly, and it takes some experimentation to figure out what works best.