If/when I notice someone who didn't reply to my VM and I talk to them on a regular basis elsewhere anyway, I may playfully remind them to go respond. If they're not responding to me, I assume they're busy, click-and-forget, maaaybe losing interest, etc. I really don't mind if someone, mod or not, doesn't respond to my VM because...
I'm sooo terrible at responding to my VMs myself. Idk how, but I am in like 35+ conversations right now. I can only respond to about 10 in a row before I wanna do something else. Then, those members reply to me on the same day. Old VMs are pushed away. I won't see the old VMs until I feel like bulldozing all of my notifications.
Staff should totally make a good effort to answer questions, or at least guide the member to the appropriate thread/forum. However, conversational VMs are usually low priority. If you get a response from me within a month, everything is still okay! :P