When you start VS, it's PATH starts with the the SYSTEM variable PATH ... However, VS now has each Project containing a property sheet containing PATH (and environment variables). Presumably (my guess) is when you creat a project, VS imports the global environment variables, including PATH, friom the system/user environment. From thereon, the user, OR some installation update (VTune, Parallel Studio) updates the project property sheet environment variables, and in this case VTune update and Parallel Studio update is truncating the extra long environment variables.
The "set PATH= truncated_path;%PATH%" would produce the incorrect results. You would end up with something like
PATH=Mares eat oats, does eat oats but li;Mares eat oats, does eat oats but little lambs eat ivy.
Please look at your installer, the VS integration part, that manipulates the environment variables. I am guessing that somewhere in there you have a fixed size buffer, perhaps containing MAXPATH, which by the way is the maximum path length for each token in PATH (IOW not the maximum number of characters in the environment variable PATH). It doesn't take too many tokens such as:
This is a real problem created by the Intel Windows compiler installers, but I have become used to it. Sometimes, an installation fails in the first attempt and one fixes the problem and repeats the installation, successfully the second time, and now %PATH% has two sets of identical substrings pointing to the compiler and VS directories. Similarly for %LIB% and %INCLUDE%.
Other vendors' installers ask for the user's permission before changing %PATH% and other environmental variables.