This is the main reason why I went away from the Vortex and I'm now on a Droid X2. For someone that probably isn't going to use it except for email, light web browsing, and texting, it's probably fine. But, I'm a power user and this phone is NOT for someone who wants to load a lot of apps, do major emailing and texting, etc.
I know that if you store too many text messages, it WILL fill up your internal memory. Also, cache is something else that can eat up space, especially if you browse the web a lot, etc.
Getting a program like 1Tap Cache Cleaner can help out sometimes. You can get it in market here: https://market.android.com/details?id=com.a0soft.gphone.acc.free&feature=sear ch_result#?t=W251bGwsMSwxLDEsImNvbS5hMHNvZnQuZ3Bob 25lLmFjYy5mcmVlIl0
I've also found that various apps can suck up space if you don't know about it. Plants vs. Zombies is notorious for that. The data files themselves take up 30mb and they reside on your internal memory. You can't move them to the SD Card.
Another good tool is Disk Usage. It will graphically allow you to browse the folders and see where the majority of your space is being eaten up. That's how I discovered what Plants vs. Zombies was doing.
You can get Disk Usage at the market here: https://market.android.com/details?id=com.google.android.diskusage&feature=se arch_result#?t=W251bGwsMSwxLDEsImNvbS5nb29nbGUuYW5 kcm9pZC5kaXNrdXNhZ2UiXQ
Hope this helps!