I have two boys so this is all theoretical for me, but when I hear of people using 'vagina' with young girls, it seems odd. Mostly because, while I learned early on that I had a vagina and what it was for, I had no personal experience with it until I was menstruating and trying to learn to use a tampon. I assure you, my 3- and 1-year-old boys have personal, daily experience with their penises, so it would be just as odd not to use that word.
I sort of like the word vulva because is is a general term for all the external female parts, but it's not commonly part of most people's vocabulary, which is what makes it 'heavy.' It's like saying scapula when you mean shoulder blade - they're both accurate and precise, but one is normal and the other is technical. And talking about wiping the vagina after going potty is like saying clavicle when you mean shoulder blade, since the vagina is interior and not part you wipe.
I don't talk about wiping a bottom, butt, tush, or anus. I just say "wipe." I imagine for a girl, when I need to specify, I would say "in the front" and "in the back."
Regarding pregnancy, I usually talk about the baby being 'in my belly.' Stomach (tummy) is a specific organ, up high right under the rib cage, where my food goes when I swallow. Kids come up with the oddest ideas anyway about - I don't want to tell him that baby is in the same place as the food I swallow. Belly is a nice general term for the middle part of my body where my uterus, bladder, intestines, stomach, etc are. Belly sounds normal to me. Womb sounds old-fashioned. Uterus sounds technical, so I use it when talking to my 3-year-old, but only when I want to be more precise than 'belly.'