gain muscle mass

An old memory from somewhere tells me that isometric exercises will allow an accumulation of bulky muscle. I don't know if it's true or not. Sadly, genetics play a roll in spite of your best efforts.

Being a vegetarian means you've got an uphill road, but that's a personal choice.

Now doctors say that the best exercise is probably one that takes a muscle through it's full range of motion, as opposed to isometric exercises, which were a fad once.

That was pretty old advice from back when I was in physical therapy for shoulder joint replacement. Might not have been useful even at the time.

hey man,
at one point of my life I was struggling with the same issue
fortunately for me I have been able to overcome that and I'm going to give you some tips.

you need to work out some more, probably improve your workout rotine (add new exersices, skip cardio, vary the weights and reps, try to do mainly compund exersices) and also you need to calculate your protein, and caloric intake.
you should be consuming 1.5gr of protein for every 2.20lbs of body weight, and same thing with carbs.
since you're a vegetarian I recomend you to take wey protein, since it comes from milk and it's one of the best protein sources there is. take a 30gr shake with 5gr of creatine 30 mins before your workout and repit the dosis within 30mins after your workout. this shakes will count towards your daily protein requirements.

let me know if you need more info or anything.

lol that's exactly why I chose it

Why you go only 2 days for gym? I think you should got at least 6 days. It is good that you take protein supplements with milk as these supplements can improve your work outs. Try to do daily work out.