Not quite.
Vegemite is made from vegetables, while Marmite is made from yeast (a fungus). Perhaps "Fungamite" would more appropriate?
Err.. Terry, I think you're not quite right there. I have some Aussie friends that kept on about Vegemite being the best thing on Earth. As I had no idea what it was back then, I consulted wiki. It said that vegemite was a yeast extract, from leftover yeast after beer brewing. If I am to believe wiki, and I usually do, marmite and vegemite would be quite similar, albeit that Marmite has a far more intense flavour, so wiki says.
I cannot say anything about taste, as I haven't come across any Aussie cats yet...
Fred