I was wondering if MPT supports using mp3 and ogg sounds as samples
No, sorry - you'll have to decode them with an external app.
and also can save in ogg format now
No, but you can easily save as wav and encode to ogg when you're done with an external app.
i made some kickass sfx on ogg format, and would like to keep them that way.
Why? It is generally bad practice to use lossy compression on samples.
If I were you I would decode that one, and save all future samples directly to a lossless format.
Also does it support multiple vsti instances again? I have a sweet synth that has like 9 presets that would sound great together.
Yes, just load the second instance into another plugin slot. It should work just fine; if not it's a bug - let us know.
Note that some VSTis can play different presets simultaneously in a single instance. Such VSTis are called "multi-timbral". Try pointing multiple instruments set to different midi channels and midi presets to the same instance. If your plug is multi-timbral it will work.. if not just use multiple instances.