There are two variants of specifying X11 resources in
.Xresources. You can either specify resources for an application name, or for a Class. Classes are typically named with leading capital(s).
I have in my .Xresources only this:
In the example
xterm, you can use the parameter
-name xxx, to change the application name to
xxx. That way you could have different "styles" of xterms, for instance.
When you specify resources for a Class, it should affect all instances of that class (all
XTerm applications, no matter the application name).
* is a wildcard. People use it to set the resource for many different widgets. Perhaps a button or a scrollbar. If you always want black background on everything, instead of having many rows of X resources, you can use a wildcard.
In your example, you should try Geometry with a capital G:
XTerm.VT100.Geometry: 80x64+240+120 to set it for all your VT100 XTerms. (but not for TEK ones).