I actually made it work with Vim, including the backward search, but I am trying to figure out how to make the forward search to work because it is the last thing missing. But for that I left Vim out of the equation and I am studying the problem by calling the command from the powershell, as explained.

I just tested the prerelease and it does not work. I also tried to build with pdflatex instead of xelatex and it didn't work either.
In this case every time I run the command SumatraPDF.exe -reuse-instance -forward-search C:\Users\myname\vimfiles\after\example.tex 10 it opens a new empty instance of Sumatra.

1. You need to provide path to PDF file i.e. SumatraPDF.exe -forward-search "<path>\foo.tex" <line> "<path>\foo.pdf"

2. You need to quote paths i.e. "foo.pdf", not just foo.pdf, so that it doesn't break when path / file has a space in it

3. You probably need to have SumatraPDF running and PDF file already loaded

4. You probably don't need -reuse-instance

5. Check the logs if it doesn't work

-forward-search converts the command into DDE command and sends it to existing SumatraPDF instance

-forward-search is meant for integration with a text editor. It tells sumatra to go to a position in PDF that matches position in .tex file. So Sumatra needs to know:

• pdf file
• tex file, line
• synctex file to do the mapping

You need to provide path to PDF file i.e. SumatraPDF.exe -forward-search "\foo.tex" "\foo.pdf"

Tested with the released version of Sumatra and it works perfectly. Many thanks! But please, update the docs.

P.S.
If you want I can add few lines how I managed to setup Vim. :)