Does pregnancy causes the vagina to swell? I am 18 weeks and 4 days, I feel like my vagina is softer and seems to be swelling. Is it normal?

Can be. The hormonal changes that occur when pregnant, can cause these changes in the vaginal area.
Completely normal. A pregnant patients mucous membranes of the vagina can become congested, beginning at about 8 to 12 weeks and continue through the pregnancy. This is driven by hormones.
Vaginal swelling. This is not an unusual occurrence. Some from hormones, some from engorgement of the tissues.