SACRILEGE! A Ford engine in a Vega? I think you have a nonfunctional tach because you're being punished by the automotive gods! But seriously, if the car runs, the tach output of the MSD(usually a male spade on the same end of the unit as the main power wires) should work. I have seen people try to hook the OEM tach to the MSD's white(- coil input) or the black(- coil output), neither of which will work.
77 Vega Estate wagon- project will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(firstname.lastname@example.org)(email@example.com 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
04 Silverado 2WD ex.cab (work truck/parts hauler)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
01 Saturn SL1- older son's ride
99 Saturn SW2- spare
and 2 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....