In general, for SuSE, you'd want to do this:
YaST2 | System | Runlevel Editor | Advanced mode= Y,
Select the service you want to enable (e.g. Xvnc) and select the runlevel(s) you want to start it at (e.g. "5", for GUI runlevel)
For non-SuSE hosts (without YaST2), you'd use /etc/rc.d and/or /etc/inittab.
I don't know if your version of SuSE/YaST2 happens to support XVnc.
If it doesn't, this link should help: