1998 Toyota Hilux new alternator leaking oil *ideas?*

Hi CarTalk Community
Lou from Aussie land here. I have a 1998 Toyota Diesel Hilux (twin cab 2 door pickup). It has stock standard engine.
Recently I did a major service at 280000 km (174000 miles) , the main reason for choosing to do a huge service was because after a recent four wheel driving trip the heater core starting leaking. Here’s what I’ve done

  • Replaced heater core (took out entire dash board to get there)
  • Replaced timing belt
  • Replaced water pump
  • Replaced starter motor
  • Replaced Alternator
  • Replaced radiator
  • Replaced all hoses and belts
    All parts new and after market.
    The problem is
    I replaced the alternator because it was squealing, stripped it right back and discovered a worn bearing so put new one in, after this it started leaking oil - took it back got it replaced put new one in again and it’s still leaking oil.
    Have checked oil pressure and it’s leaking from the vacuum pump into the alternator through the shaft seal.
    My question is why is it doing this?
    I’m stumped.
    After ideas please and will do my best to answer questions.
    Thanks y’al

You just said it. Oil is leaking from the vacuum pump. Fix that leak.