Since my mother-in-law has lived with us, she has been able to save money because her expenses are minimal. We pay for almost everything. I want a break and my husband does not feel that his mother should have to pay for any care for a week or two so we can go on vacation. She can afford it.
If it's the 'not feeling right', that's trickier. Does he feel guilty about leaving her to someone else's care, maybe, too? In that case, once you've shown him the black and white answers, you'll have to change tack and focus on how important it is for you two to get proper rest so that you can continue to look after MIL. Best of luck, hope you see some progress soon.
Your hubs (& his mother) are being unreasonable in their approach that the caregiving and support that your household provides should be for free to MIL. The issue isn't just that you want a vacation & your own time away with your hubs, but a much bigger issue of how to equitably provide caregiving and taking your concerns and feelings in this to be considered.
You know hubs & his mom best.....are they old-school in which the daughter in law is subservient to mil and he rules by being the male head of house? Or it is more that he - now that mom lives with you all - has gone back to moms in charge as it was when he was a kid? If there are siblings, where are they.....if they exist and avoid mom like the plague, that should be telling you something. Are you self sufficient financially or are you dependent on his income? Do you & hubs work outside of the home? Do you have kids & grandkids?
Although I'd personally take Pams approach to let hubs/MIL sort it and take my own vacation, this may not be feasible if Wanda is not self sufficient financially & his mom is a manipulating clever old battle axe.