iamarche,
Authorize.Net works with Joomla 1.5.3 and VM 1.1. I am using it myself to process Visa, Mastercard and Amex. One of the things I would advise you to do is start over and check every single field, including the Transaction Key. One of the weird things about the Authorize.Net configuration in VM is that its not very sticky... if you move from one screen to the next it may not save what you entered. So fill out a field, save. Fill out another field, save. Don't move from one screen to the next until you have saved or submitted after every entry.
If you need some more help, send me a PM and be as specific as possible... I'll do what I can. You'll probably have to go to Authorize.Net and request a new transaction key though as they don't last very long.
Regarding VM Support... its not that its not there, but it is difficult sometimes. People like to come to find answers but once they find them don't update their posts and often just move on to something else. Also many users who find answers don't bother to come back and help someone else. I have many posts up here that for whatever reason have not been responded to, but the best way to fight that is to help others when you can and hope someone will take notice and help you when they can. Its a community and its starts with each of us as individuals. Sometimes you have to give back before you can receive.