First off, don't place too much value in your identification as an ectomorph. It belongs to a very old, and debunked myth about somatotypes.
Now, the point of creatine isn't to gain a permanent weight increase. The point of creatine is to improve muscle recovery between sets by increasing their susceptibility to water, and thus their durability and stamina.
Another benefit of creatine is that you will likely experience increased energy levels during workouts, because of this recovery.
What creatine does NOT do, is provide permanent gains. You take creatine to maximize your workouts, and make better gains while under the effect of it. Then, when you stop taking creatine, you muscles will still have made those gains. You will lose the water weight (which I think you're confusing with going back to your "old form"), but the gains you made are there, unless you stop training all together.
As you probably know, meat and fish are the best sources of creatine, and even then, you'd have to get 500 grams per day to get the optimal amount of creatine. What you, as a vegetarian, should do, is get enough of the amino acids arginine, glycine, and methionine which are used in production of creatine.
Some foods that are rich in glycene: Raw seaweed, spirulina, raw watercress, spinach, soy protein isolate, sesame seeds.
Some foods that are rich in arginine: Peanuts, walnuts, coconuts, soybeans, chickpeas, oats.
Some good sources for methionine: Brazil nuts, oats, sunflower seeds.
For extra reading, I recommend this page on vegan health and weightlifting.