If possible, I recommend you save these values in gconf (this is how Hildon remembers what desktop you were last on when you reboot, too, for instance), and quite a lot of apps store their settings through gconf.

Alternatively, a simple file in the same folder as the rest of the vkb's data would work too.

Also, what you could do is use three fields/variables/gconf-entries/lines-in-a-.conf-file etc:
1. Lock
2. Landscape
3. Portrait

If you make it use integers or a name string instead of a boolean for 2 and 3, it'll be more scalable to other 'types' of keyboard layouts - IE, if someone writes that swirling/circle input method as an add-on to your vkb and submits it to you as a patch or something, those variables no longer have to be reworked.

And then the lock variable can be a boolean, and it would be settable in the settings somewhere (toggle/radio button), that prevents rewrites of 2 and 3. That way, you can set the default so that even if you normally like the qwerty, but for some situation chose to use alphanum one day, and close out of the keyboard without switching back, it'll first check if you have the lock variable set to true, and if you do, it'll leave your 2 and 3 variables alone, but it'll edit them of lock is set to false. And in the settings, it can simply be called "Lock Default Keyboard" or something.

But it's up to you. Ultimately, most people will be just fine without this extra locking (myself included) - it just gives an extra layer of soft cushy convenience for people.