I usually incorporate a bit of "extra" to the bottom of the master (or 'buck' as you've called it.) This extra is delineated with a scribed line, which usually 'transfers' into the molding so I can easily trim it. The extra allows for a "pry surface" that does not intrude on the mold area so if there is any minor damage to the master, no worries.
Also, make sure not to create an "undercut" situation, or at least not anything other than very minor. For example, the F-16 canopy has a cross section that is greater than 180 degrees around (which is why Hase & TAM kit canopies, among others, have a mold line across the top - a two part mold is required!) Vac molding such a shape means the part essentially traps the master, so some means of freeing it is needed. For the F-16, the plastic is flexible enough, plus the profile (long dimension) is NOT undercut, so just flexing the sides outward will be enough to free the part.
Vac molding does not require release agents; just make sure your "engineering" of the master is paying attention to the "trapping" issue and you should get good results. Part of the fun with vacforming is the experimentation, so expect to have some 'scrap parts' as you improve your technique.
Now, the above discussion is most relevant to molding canopies and small contoured parts. If you are molding major components for say an aircraft model, then the "extra" I refer to may actually not be indicated since the idea is that the part will be separated from the 'sheet' at the juncture of part and sheet, so some precision is required. Then it boils down to avoiding undercuts that would trap the part - unless you are working with complex, multipart molds!
Think of it this way: If I wanted to mold a sphere, I could in theory create a female mold with a tiny opening to "suck" the plastic into the shape (this is how milk jugs are formed, in fact, but by "blowing" with pressure, not using a vacuum.) However, the part would be "trapped" in the mold unless I can split the mold apart along the equator-line (again, actually done with blow-molded parts.) So, instead, I will split my master part in the middle to have two half-spheres, attach them to a flat surface and viola! a master that has no undercuts because the widest dimension of the part is facing the plane of extraction. This principle applies to all molding, whether you are using female or male molds, and is the same concern for injection molding, casting, etc.
One extreme is when the "lost wax" process is used to cast intricate brass parts (such as those beautiful cast metal landing gear sets.) In lost wax, the master part is made of stiff wax (jeweler's wax) which is then encased in a block of poured plaster-like material. Once hardened, the ceramic mold is heated to melt out ("burn out") all the wax master. Then the molten metal is poured into the mold, cooling into the cavities and forming the part. Only problem - the part is completely trapped in the one part mold! So, the mold is broken apart to extract the molding, so not only is the wax "lost", but so is the mold - which explains why such parts are pricey. Molds must be made everytime a part is cast! But such detail, ah!
Hope this discussion helps with your project.
Regards, Robert Beach