Bushgirl
I look after Tarptent in Australia.
Take a big breath and relax because this is going to take a while...
First a direct answer : all Tarptents can be fully sealed against bugs , that is the tent part of the name.
All Tarptents have some vestibule space; there are some very good photos as well as 3D views and diagrams on the site so that you can see exactly how much space you have for yourself and your pack.
(go to a product , say Double Rainbow and you will find a button called specifications as well as the video one)
Now, here is a bit about the Tarptent way.
All of our shelters set up dry. Regardless if they are single or double wall (about half are double wall...) you set up the fly first or both together.
All have 360 degree ventilation as well as vents and ways to keep door/s partially or fully open in the rain.
If you watch that MSR video on condensation (please do...) you will see that indeed ventilation is the enemy of condensation however you will also discover that contrary to popular belief the second wall does not eliminate condensation it may just keep you away from it..
Now if folk using a single wall tent got wet every time (or most times) they used one was the case, well we would not have single wall tents.
By we I mean tent users , not TT...
Again contrary to the way some think , the role of a backpacking tent should not be to keep you warm but to keep you out of rain,wind and bugs .
However many users will shut themselves inside a tent all zipped up and what happens ?
As the temperature inside raises , humidity will increase , that will dampen you bag and clothing so later you will feel cold.
The Tarptent way is to allow maximum ventilation so as to get rid of as much condensation as possible regardless if you see it or not.
Here is a practical example.

This is up at Mt Buller.
I spent the night inside my Notch, a mate inside his Scarp 2 , a couple of guys had a well known 4 season tunnel tent and another guy a small "4 season" solo double wall shelter (both well known here in Aussie)
We all had decent mats (very important)
I had by far less to eat then the rest (however enough for me...) and was about 20 years older than the rest.
To top that , I was the only one to have a zero rated bag, the others had -10 to -15 bags.
Half a way through the night one of the guys in the tunnel tent (winter type ,double wall) bailed out and went inside the hut (still heated..) .
The next morning turns out that my mate in his Scarp had a reasonable rest but could have done with a better mat, the others had a cold night.
Why? their tents had the inner and their bags saturated with moisture, I had a little bit of dampness on my foot box, even so I had a great night.
That was not an unusual experience for me...
BTW, the Notch is a 700g ,3 season TT shelter...