Many people do tap them into an existing line. BUT, I have previously had a BOV tapped into the boost gauge line and it made the boost gauge flicker spastically when the BOV was opening and closing so I thought it may actually not be a good idea to have the vaccuum line for the BOV plumb into anything that needs to read an accurate boost/vaccuum.
The golden rule though is to keep the line as short as you can (being practical about it of course).
I would use the dedicated point you mentioned at the back of the plenum and use a commonly available reducer