Step 8: Adapt your vacuum cleaner hose to the nipple
You should probably take your vacuum cleaner hose to the hardware store, and find some plumbing fitting that adapts 3/4" pipe threads to something that fits your vacuum hose (inside or outside) reasonably closely. Then you'll have less shimming or packing or taping to do.
(I got lucky. My Shark vacuum's hose fits perfectly and snugly inside a fitting meant for 1" unthreaded pipe. So for 94 cents, I got a perfect adapter---a PVC pipe elbow with a 3/4" threads on one end and a 1" socket on the other. It took a few minutes to find it in the plumbing department at Lowe's, and about 30 seconds to install.)
If you're using a shop vac with a standard large (2 1/2") diameter hose and an adapter to the standard small (1 1/4") diameter, the adapter will likely fit right over 3/4" pipe nipple, with the pipe threads going inside the (unthreaded) adapter. (That's not how it's designed to work, but it works.) You can just screw the adapter over the nipple, with epoxy on the threads and around the outside where they meet, to permanently connect them and seal the joint. (If you don't want to commit to permanently joining them, you can pack the threads with tacky putty, screw them together, smear a little tacky putty around the outside of the joint, and wrap duct tape around the whole mess.)
For most household vacuum cleaner hoses, the hose will fit loosely over the nipple, and you need to shim it out a little so that it fits snugly. A few wraps of duct tape may fill the gap. If the gap is very large, you may want to wrap some craft foam or leftover weatherstrip around it first.
If you have an extra vacuum hose attachment you don't mind sacrificing, you may want to cut it down and epoxy the base of it to the pipe nipple, to make a nice special-purpose adapter. If the gap is small, say 1/16" you can just fill it with epoxy. If it's larger, you may want to make a shim out of some appropriate-thickness piece of flexible plastic. (Or a piece of similar-diameter tubing, slit and wrapped around the nipple, epoxied in place and smoothed over at the seam.)
In figuring out how to kludge this together, keep in mind that
(1) if you're only using a vacuum cleaner for suction, you don't need an absolutely perfect seal---a little seepage is not a problem for a vacuum cleaner to keep up with
(2) if you want a very good seal, you can use something soft like tacky putty or even modeling clay, applied to the OUTSIDE of a joint that seeps a little, and it will make a great seal. A vacuum leak sucks the squishy stuff inward, sealing the joint better, rather than blowing it off. (Tacky putty is nicer than modeling clay because it's less greasy and easier to get off, so it's less of a problem if you decide to do something better later. Either will make a great seal. Rubber cement can be a good sealant too, and it's not as hard to peel off as silicone, much less epoxy, if you want to re-do the joint later.)
(3) Silicone seals very well, but thick layers may not cure in reasonable time. (Moisture in the air is what makes it cure, and moisture takes a long time to diffuse through thick silicone.) Don't fill large gaps with it unless you want to wait days for it to set, and don't expect even thin silicone sandwiched between impermeable materials to set up quickly.
(4) Rubber cement doesn't fill gaps as well as silicone, because it shrinks as it dries; it's fine for a sealing layer around the outside of a joint, though.
(5) Duct tape alone often works fine, temporarily. It works best if the two surfaces at the joint are flush, so that you don't get any wrinkles across the joint. You may want to shim the smaller-diameter surface on the outside, to bring it up to the same diameter as the larger one. (A bunch of smooth wraps of duct tape on the narrower piece may work, followed by a smooth wrap of duct tape around the joint. Then you can detach the parts by unwrapping that final wrap, and put another final wrap across the joint you reattach the pieces.)
(6) If you're making a permanent adapter, and the threads on both ends of the nipple are not helping, you can use a longer nipple and cut the threaded part off one end with a hacksaw. (PVC cuts easily with a hacksaw.) Then you won't have to pack the threads to keep them from creating a leak.