Personally, I've never heard of adding vinegar....maybe someone will tell me what is the reason for this? Obviously it will make the water acidic, but...?
A few other questions first....has the pot got drainage holes as you say you cant tell if it's ever overwatered?
It lives on the window cill....is it a sunny aspect, how much sun does it actually get, is it direct or filtered sun?
I suggest you uproot it, check the state of the root system and re-pot in fast draining gritty soil, water once, just enough to settle it in, place in a well lit but not in direct sun and leave alone for a few weeks and see if it recovers from whatever is ailing it.
I'm sure someone else will chime in with other ideas, so hold on until they do.

Gill UK

Is it just me or does anyone else think the soil looks awfully dry?

I've heard that you should add vinegar to the water if you're using tap water..to level out the pH or something?

This pot doesn't have drainage holes..Perhaps I should re-pot it.

It is very sunny and warm on my window sill, it gets full sunlight.

The soil looks dry, but I watered it not two days ago. I usually water it once a week or so. I'm always afraid of over watering, but the plant doesn't look dried out to me either.

Oh, and I forgot to mention that I have only ever misted/sprayed this plant..I've never poured water into it. I know that this type doesn't like too much water on its leaves, so I spray around the soil.

spraying is bad honestly. You want to thoroughly water until it comes out the drainage hole then let soil dry completely before watering again. I don't see much perlite in your mix so be careful not to water too frequently.

If you have only been misting, I'm leaning towards you underwatering and that's why leaves are limp and wrinkled. Plant looks healthy otherwise. Keep sunlight to a maximum and give it a good drink. Sounds like your root zone is not getting enough moisture to me.

Good luck!

no drainage hole = death. i would def repot the plant. Cut soil with 50/50 soil and perlite and put into similar sized pot with drainage. Give it a good drink and let it dry out completely.

You got lots of good advice from all posters.

Well draining potting mix is very important for succulents. Search this forum for 'gritty mix' - to get idea how to mix it. Should you have problems with getting ingredients, at minimum mix in perlite (at least 50% of the mix, could be more). Perlite is easy to get at many stores.
What kind of soil is it in now?

Drainage - already mentioned.

The pot seems to be slightly large - especially if you have water-retaining soil, so lots of soil will take long time to dry up. I would plant it in smaller pot.

Don't add any vinegar to your water unless you know ph of it (test). If you are concerned about your tap water (it's probably chlorinated if you have municipal water-not so great for plants, but they will survive), you are better off to buy some distilled water in large jugs & use to water your plants. Better yet, collect some rainwater (it is naturally little more acidic than tap water), especially if you have lots of plants.

Simple way of checking if it needs watering is to use a skewer; insert about half-way deep into your pot (or little more). If it comes out 'dirty', soil is too wet (just like checking cake for doneness, lol). Don't water until skewer comes out clean. And water soil around the plant (already mentioned).
Rina

I think your succulent is a Tacitus Bellus and if you follow the advise you have been given it should bloom. I love this succulent mainly because of the blossoms. Good luck.
Dottie

Also, if you allow your water to stand overnight, much of the chlorine will be gone. Just like the situation with tropical fish; when you change the tank water, you allow it to stand overnight before placing your fish back into the tank.

Wow, thank you all so much! I will follow all of the advice you've given me.