As pointed out by Michal and e-latour on Aros-Exec, I've done a silly mistake in ConfigIP, which saves network preferences in ENV: instead of ENVARC:. This is like clicking on 'use' rather than 'save' on the other preference programs, and new config will be lost at the first reboot.
In order to correct this, a newer version of ConfigIP will be released really soon. Installation is pretty easy: just download the zip file, expand it to your host's VmwAROS directory and mount the ISO in VMware as CD-ROM #0 or #1. When the ConfigIP CD icon appears on AROS workbench, just open it and drag to HD0: the Prefs drawer. That's it: now you can effectively configure your network in a easy way.
EDIT: I'm investigating a problem that blocks ConfigIP from changing the IP address on the fly as expected. If you have any clue you can suggest it into this aros-exec discussion. In the meanwhile, in order to make ConfigIP running on your VmwAROS distribution, just open it with the Editor and change every 'env:' you find in 'envarc:', then save. Once done, just run it again, save your preferences and reboot AROS.