Q:

Where do the major space battles in Star Trek: The Motion Picture take place?

Star Trek: The Motion Picture mostly seems to take place in interstellar space, with brief stops at Vulcan, Earth and prolonged sequences aboard the Enterprise.
But where in interstellar space does V'Ger fight the Klingons? Where in interstellar space does the Enterprise intercept V'Ger?

A:

The excellent Star Trek: Star Charts gives us the route taken by V'Ger through Klingon space and then into Federation space. The initial conflict with the Klingons seems to have taken place near to Khitomer, just inside the Federation/Klingon border, hence why the Epsilon Monitoring Station was able to detect the event.
V'ger then proceeded toward Earth, passing relatively close to Andoria and Vulcan, before finally encountering the Enterprise somewhere between Vulcan and Earth.

Red Arrow = Initial conflict with Klingons
Dark Green Arrow = Epsilon Stations
Lime Arrow = V'ger stopped by Enterprise.

