I have worked with glass, quite a lot, I used to repair and make stuff for lab use. Even with hours of practice I was never very good at it but some of the stuff I made was OK.
What I can tell you is that it is a lot more difficult than you think, a LOT more. Even with small tubing (1/4") it is difficult to apply heat evenly whilst maintaining the structure, large tubing gets very hard to handle, and this is why the professionals use 'glass blowing lathes'.
From what I've seen of evacuated tubes, it appears to look like one 'test tube' inside another, and then these are sealed together at the rims. To make something like this you would first make the inner tube, sealing one end and blowing it to make nice round end. The outer tube would be blown the same way and then a small tube joined to the round end. I'm not sure how to join the tubes at their rims, it's been a long while since I did any of this, but I can guarantee it would be a sod to do; keeping the two tubes parallel being the problem. I think this used to be done by wrapping the inner tube in card which was burnt away later with acid. Next you'd have to anneal the joint to remove the stresses.
Lastly the structure would be evacuated through the small tube which is pulled closed and fired off.
You'd then have to hope and pray that all your hard work wasn't just going to implode.
Personally, I wouldn't bother.
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