The truth is there is no direct substitute for vegemite. If you are in the UK then you could use marmite which is a slightly sweeter version. If you want to make the pies a little Asian then you can use miso which is FULL of umami. Worcestershire sauce is also a nice addition too. Vegemite is EASY to find here in Australia and has a long shelf life so I would advise to grab a small tub and try it out!