Major changes in version 2016.10.01:
Download vDos: Installation program 2016.10.01 | Source files and older versions
First read the little documentation vDos has to offer to make it run your application! Keep in mind you once had to configure DOS, the transition to Windows (XP?) took time. You forgot about that…

Have a look at the FAQs page, consult the vDos forum at SourceForge, eventually post your question/problem there. The forum is monitored by me and Edward Mendelson, replies are mostly real quick.
If your issue seems specific to your situation/application, or you don’t want it public: Send a mail to (change the "a" to an "o").

Submitting a problem or issue: Describe it as detailed as possible, eventually attach your autoexec and config.txt files. A mere “My DOS program won’t …” doesn’t help to come with a solution.
Calling 4DOS from within a program (shell command), vDos assumed the required free memory was the 4DOS executable image size. But 4DOS needs a bit more, so vDos could conclude no memory swapping was needed. In rare situations preventing 4DOS to load with insufficient memory (as it wouldn’t in “real” DOS).
With swapping, CPU registers were incorrectly restored on reentrance (documented DOS behavior, but…).
If the extrabytes value (last page size) of an executable header was 0, vDos didn't load the actual last page (512 bytes).
Due to a Windows bug, opening a file could return an incorrect error code when failing. If a DOS program explicitly tests for that code, it could go wrong (so far only reported with DataPerfect).
The Windows API GetBinaryType function doesn't always return correct information for DOS executables hosted on non-Windows filesystems. This could cause vDos to let Windows handle those DOS programs.
Establishing the "real" keyboard scan code, could result in an incorrect scan code being reported with specific key combinations and non-US keyboards.
The CPU SHL byte-wise instruction didn't set the carry flag if needed. SAR was also incorrect due to a typo.
Some protected mode programs were not supported anymore.
The CMD command is enhanced (see 4DOS help text).
The internal print processor also supports basic PCL printing directly. The bare SEL option is dropped, displaying the Windows printer selection dialog has become the default.
A SPOOL option is added to circumvent possible persistent print time-outs (or to collect DOS print jobs).
More accurate horizontal positioning, Windows fails in updating the position correctly at printing multiple characters.
To improve readability, the vDos window has some extra white space (half a character) to the left and right. This can be turned off by the optional leading minus sign in the WINDOW = [-]… directive in config.txt.
The internal font is replaced by an optimized/customized Cousine font. It now supports more characters (code pages) than any other font, and is also the default font for printing.
Do you actually know what you're about to download/install? Anti-virus program complaining: Read (at least) the FAQs page: Getting started - Anti-virus program blocks vDosSetup.