So say you've collected the first pulls of your acetic acid/water wash. Not sure about you, but my sense of personal safety is inversely proportional to the volume of highly caustic liquid Im handling.

To substantially reduce the volume of water, would it be safe to add MgSO4? MgSO4 has a molar weight of about 120g/m, and can hold up to 7 water molecules, which has a density of about 18g/m. So each gram of MgSO4 should hold about a cc of water, right? Then you would just filter off the salt, and go on about basification/non-polar extraction.

Much faster than boiling off or using a dehydrator to remove water, and safer/easier to handle the reduced volume of liquid.

Is this correct?

Um, no. With an excess of H2O, the magnesium sulfate will just dissolve into the H2O. It's used for drying residual water in solvents where there are very small amounts of water dissolved in the solvent, not where the solvent is water based such as an acid.

Wouldn't enough of it form the excess h2o into MgSO4·7H2O, which could then be easily filtered off?

Just boil it, as long as you don't stick your face over the pan there's not too much to worry about safety wise, even then the vapours aren't going to be caustic enough to cause any real damage. The acetic acid evaporates along with the water, i don't know if the concentration ever increases to a dangerous level in a pan as it reduces, i doubt it, but don't take my word for it. Its going to be boiling away, acid or no acid you wouldn't want to spill it on yourself.

The above assumes you're using it at a the same concentration of vinegar, so about 5%.

Don't try the epsom salt method you've mentioned, it wont work, it would be a shame to waste/end up with contaminated DMT. Just going to give yourself more work and use more solvent to get a pure product.