For instance, can I flash a custom kernel meant for ZVD over the stock factory rom that's also on ZVD? I didn't know if the kernel might take or not or whether a custom rom would be needed.
I'm not 100% positive on the LG's but on most devices you can flash a custom kernel over the stock ROM. Some goodies baked into the kernel may not function because the stock OS (ROM) doesn't support it but the basic means of the custom kernel should work.
I would allow some LG natives to speak up before taking my word 100%.
Naturally make yourself a Nandroid backup in your custom recovery before you make the changes so in case something does go wrong, you have a working backup to return to
Yeah, you should be able to. I believe the only feature you'll be able to take advantage of is overclocking though. You can download No-Frills or SetCPU to change your frequencies once you flash your kernel. Also, make sure you are flashing a kernel appropriate for the Android version you're running.