To me, it’s no more of a demand that if you were standing in front of a person and gave them the invitation orally. If I ask you, “Would you like to go to the movies on Saturday?” is silence an acceptable response? Or is it rude? There’s nothing that changes the rules of social discourse just because the invitation is on paper instead of out of someone’s mouth: You are invited to do something, so you promptly advise the inviter whether you will be able to do it or not.
Moreover, a “regrets only” rsvp is not sufficient for a situation like this one, and like the one my family just had, where the event is in a restaurant and you need to know precisely how many people will be there. Since people who ARE coming are more likely to respond, and it’s the people who are NOT coming who are more likely to just ignore the invitations, it is not practicable to assume that silence indicates attendance.
Something to consider is that a lot of people experience shower and other wedding-related invitations as being hit up for gifts, preferably gifts more expensive than the meal. It sounds like the invitee list was not very well thought out. Is it even appropriate to invite people (the Vermonters) you don’t expect to attend to a shower? This could have been seen by others as an attempt to cast a wide net for maximum loot, rather than generous hospitality. Shower etiquette has typically been that showers are not held by family members for this reason.
I think you and some of the recipients are confusing the courtesy wedding invitation with something that applies to showers. Sometimes wedding invitations are sent to people you just don’t expect to attend, as more of a notification. But this “courtesy” doesn’t really apply to an occasion whose focus is gift-giving.
IMHO, the OP is not on any particular etiquette high ground. She should give the caterer the best number she has, handle any variance as politely as possible, and consider her future guest list more carefully.