Responding to the issues that you have raised in Q&A format:

"When you lose weight by dieting , reducing it to portions to reduce the calories,do you lose just the fat or the bone and muscle too ?"
If your daily calorie intake is less than your BMR then your body responds by first using glycogen (a storage form of glucose, found in the liver and muscle), then fat, then by breaking down muscle protein. A long-term daily calorie intake less than your BMR is not sustainable - it is called starvation. I strongly suggest that you see a dietitian about this.

"Does the body leech out bone to keep Calcium in the blood constant ?"
Yes, this is controlled by parathyroid hormone.

"Can you name a few less calorie calcium food sources ? "
A dietitian, taking into account that you are a vegan, will be able to advise about this.

"I read on some sites that if you dont take the right amount of calcium daily,the body takes it from the bones to maintain blood pH. Is this true ?"
No - blood pH is controlled in a way which is essentially independent of blood calcium levels.

"Also which is the best type of protein for muscles ?"
There is nothing special about the amino acid content of the many muscle proteins. As long as you eat a range of protein-rich foods there is no need to eat any supplements. At risk of repeating myself, a dietitian can advise you about this.

"drug called Orlistat"
This drug works by inhibiting gastric and pancreatic lipases, the enzymes that break down triglycerides in the intestine. When lipase activity is blocked, triglycerides from the diet are not hydrolyzed into absorbable free fatty acids.

What this means is the fatty acids from the fats in your diet are not absorbed. A far simpler solution to this issue is to reduce the fat content of what you eat - a dietitian can advise you about this.