Ah that would be the problem I see... because it asks for a password in the current terminal? In that case, you could try to look up vncviewer's help for a password switch. The only problem with that would be that your password could be visible.
Another solution is to launch another aterm that then launches vncviewer - probably a better solution so that it won't lock up the current script. Look in ~/.fluxbox/menu for examples.I haven't used vncviewer in DSL, but in other distros you can create a password file with the vncpasswd program, and then use