We want to surround ourselves with as close to an unbroken bubble of mass loaded vinyl as we can get without interfering with the vehicle's function. There are going to be openings in the barrier. We want them to be as small as possible. That doesn't mean leaving a 1" gap and running a strip of duct tape over it. Unbroken means at least 1 lb/ft² every where.
The strongest way to join two pieces of MLV is to overlap them and join the overlap using HH-66 Vinyl Contact Cement. I almost always take this one step further, cutting a 2" strip of MLV two overlap both sides. Doing this requires less precise alignment and it helps to hold the shape of assembled pieces.
This is the spare tire well in a 2010 Toyota Prius. MLV is very flexible in one direction at a time but when trying to cover compound curves we need to assemble pieces. If you're wondering, heating MLV will not make it conform to complex surfaces - I've tried. As you can see in the photo, there are gaps between the pieces. We don't want gaps! Sound passes through them and those pieces aren't going to stay in place.
I cut 2" wide strips of MLV for this purpose. Combined with HH-66 Vinyl Contact Cment it makes a permanent "tape" that solves the problem.
Press the MLV "tape" over the seam. The bond is strong enough to work with immediately and will be stronger than the MLV in 24 hours. This technique lets us be less than precise with our seams and guarantees no break in our 1 lb/ft² coverage. Assembling these pieces over a compound curve lets us keep the MLV as close to the sheet metal as possible.
A nice feature of HH-66 Vinyl Contact Cement is that it isn't sticky at all when dry. That means the ooze you see on the edges won't stick to anything you put on top of it.