How do I start vassal in Linux?

I do not think anyone is too old or computer-illiterate to use Linux (and in particularly not Ubuntu).

Did you read the Installation Instructions? In particular the section

If you are uncomfortable with using a command line, you can use the graphical installer Synaptic to install the JRE. Just

  1. open Synaptic
  2. search for default-jre
  3. click install (you will be prompted for your password)

After downloading VASSAL for Linux and you double click it, you will likely have the directory VASSAL-3.7.18 in your ~/Downloads folder. Move that somewhere more permanent, say your home directory ~/. You can navigate to that folder and double-click the script VASSAL.sh. If that won’t execute, right-click and select Properties, then enable Executable as Program.

If you want to integrate VASSAL fully into the desktop, follow the Desktop integration instructions.

In the hopefully not too distant future, you will be able to download a Debian package from the the site and simply install that. That will give you full installation and integration with minimal hassle. You can get a preview from here (download by clicking the link, then double-click the file vassal_3.7.19-SNAPSHOT-e8de2b8-1_all.deb in your ~/Downloads folder to install - you will be prompted for your password).

Yours,
Christian