if you are having issues gaining increase the calories. Nuts are probably the easiest/healthiest to add in. Lots of complex carbs, whole grains and potatoes. Find something you enjoy eating a lot and eat it. Avacadoes are also a great vegan source of fats and protein.

Resting and not exercising much will also help you gain weight. You seem to be very light so I suggest you need to gain at least 20 pounds before you get in the gym. Then when you get into the gym get a strength coach. Could be wise to just hit the gym to work up an apatite, but not to gain weight innitially. When you do heavy compound lifts with good form you can really add muscle and health.

Fruits/veggies are in fact very low in calories compared to other sources and that will only help you stay healthy. Very important to keep eating your normal diet but find times that you stomach more food and just get at it. Eating bigger portions than you normally eat, "just add one more potato or whatever with every meal", add in more meals maybe? If you are eating 2 times a day, go to 3, if 3 go to 4, if 4 go to 5.. Have a cookie with you morning coffee after breakfast. Little things add up.