T off the brake booster line. It's 7/16id
most standard vacuum lines are 3/16id
so you would cut your brake booster line stick a brass T fitting with whatever pipe thread you like... most vacuum blocks are threaded for 1/8npt so better to get a T fitting with that thread on one side... the other two sides of the T fitting should be 1/4NPT... put two 1/4NPT to 7/16ID barbs on two sides and a 1/8NPT to 3/16 barb(or whatever size barb you are using to match your vacuum line size). All your vacuum needs should come from the same source but isn't necessary.
better to do a vacuum block...
like this one