If your VMware virtual image is 15GB in one file, every time you fire up the virtual machine, the image will change. Time Machine will have to make a new backup for it, which will ultimately result in ~15GB or more being backed up every time you run Time Machine. Every day, every hour.
Obviously, this is very time-consuming. Backups will be faster if you have an external FireWire 800 drive, but it still takes a considerable amount of time. And CPU – my system becomes very unresponsive when a large backup is made.
If you can afford to keep a separate backup of such a huge file that constantly changes, then by all means, do that, and exclude it from Time Machine. For example, you might not even want to maintain an hourly backup of your VM because it's not that important. In this case, it'd be easier to back up the file separately.