1. Try setting your TERM environment variable to "wy55". If that doesn't work, try "wy50". I know the WYSE WY-50 is supported, and that should be close enoough to the WY-55 model to work. At least, it will work better than the VT100.
There are two databases of terminal support information in Linux: termcap and terminfo. Termcap data is in /etc/termcap, and terminfo data is in directories under /usr/share/terminfo. If you look in these, you'll see that there's a huge number of supported terminal types. If you find that the wy50 setting does most things right, but a few things are still broken, and you have docs on that terminal that tell how to program those features, you can create a termcap or terminfo description for the wy55 that does things just right. Like you really want to get into that! :-)
2. It's possible that the system didn't see that serial line. Figure out which device it is (one of /dev/ttyS?) and echo something to it to see if it shows up on the WY-55. If not, try another ttyS? number until you get the right one. Then you have to arrange for a getty program to be running on that device so that the system will present a login: prompt on the terminal. This is usually done in /etc/inittab. You'll see some mgetty lines in there for the virtual terminals running on your system's console. You can add similar lines for the serial device your terminal is on.
Hope this helps!