I have a similar problem to you but am maybe about 30 minutes ahead of you on my journey of discovery and am currently looking at getting weight down, which means reducing volume as well if I want a lighter rucksack. Here are my investigations so far.
My sleeping bag (Snugpack tactical rated to -7 and synthetic so probably a similar volume to yours) and tent (Vango Banshee 200 the same as yours) will fit into the main compartment of my Sting rucksack which is sold as 55l. The Sting is a very, very heavy pack and so is an obvious easy gain in terms of weight saving for me so I need to think about volume a little. Here is a photo I've just taken of the tent and sleeping bag side by side in the rucksack, the tent is packed in its bag and the sleeping bag is completely uncompressed:
Now I suspect that, perhaps, your rucksack may be a little less wide than the Sting as that is the current trend so you might not get the two things in there side by side. However, it should be longer and my tent comes maybe 3 - 6 inches short of the top of the main part of my rucksack. So, perhaps if you compressed your sleeping bag it might go in the bottom with the tent stood up on top?
To give you some idea of my volumes here are pictures of my sleeping bag (uncompressed) and tent. As you have the same tent this should give you some feel for the scale of things:
So, having seen your problem I thought to see if the sleeping bag would compress enough to go horizontal in the bottom of the rucksack with the tent above it standing "upright." First of all here is what they look like alongside the rucksack but uncompressed:
My sleeping bag will go into the rucksack across the bag rather than straight up. So I set about compressing the sleeping bag a little and here is what I ended up with, this was without too much effort put into the compressing and I could probably have done a tiny bit better:
So, now we go for the bag. I put the sleeping bag in the bottom and it filled most, but not all, of the bottom of the rucksack leaving maybe 3 - 4 inches of width free, then I stood the tent up on top of it. The view from the top down the bag looks like this:
I'd say there is room for at least one, and probably two, more similar tents to stand up in there. Because the bag is falling away from the tent under gravity this is what it looks lying down, so you can see that I could easily close the lid. The tent and sleeping bag are in there in the same arrangement as above:
What I also discovered was that there was enough width remaining right to the bottom of the bag that I could actually push the tent right down into the bottom beside the sleeping bag. This seemed to leave quite a lot of space so I took a 10l dry bag and stuffed it with "stuff" and it easily fits in the free space:
In fact there still seemed like a lot of space so in went a 5l fully stuffed dry bag:
There are still plenty of little voids between all the bags and tent so I figure a couple of 1l bottles or a fuel bottle plus tent pegs, gloves, hat and that sort of stuff could still easily fit in there stuffed down between the bigger things. Also once I push things down a little and look carefully I'm still really about 4 inches short of actually being full to the top so I managed to get waterproofs, cook gear, mug, stove and so on into that space.
After all of that I have sleeping bag, tent, a 10l dry bag, a 5l dry bag, waterproofs, cooking pots, stove, mug and other odds and ends all in a 55l bag. I would say I have 20 litres of extra stuff over and above the tent and sleeping bag in there and the bag isn't completely stuffed full to bursting nor is there anything even close to coming over the top of the main fabric of the bag.
Then I went and got my 33l bag. I suspect this is probably actually 23+10 but don't know for sure. The problem was that the compressed sleeping bag would only barely fit through the top opening and when I stood the tent on top of it then it was far too long for the bag. However, if I folded the tent into a rectangle rather than rolled it I'd guess it would fit. Far from ideal I know but an interesting exercise.
So, that's been my experience in experimenting with rucksack volume using similar gear to what you have and a rucksack with a main compartment volume claimed to be 55l.
So here are some hints and tips for you:
When putting the sleeping bag into its compression sack stuff it in randomly, don't roll it or fold it as that will never compress as tightly as one you randomly stuff in.
When putting your tent away use the poles as a guide to the width you need - put the poles into their little sack and fold the tent, that is to say along the long axis so you are reducing width but not length at this stage, to make it just about the same width as the poles at the very widest part. Then put the poles right in the middle and fold it in half so the poles are right at the fold. Then start rolling it up. I've been surprised at how successful this method has been for me.
