I haven't been to Vinales to my understanding its a small town. Two of my buddies went this summer and didn't like it.
Yea a casa particular near hotel nacional is fine. The Nacional is located about 1-2 blocks west of 23rd avenue and close to linea which is great for taxi collectivos.
23rd avenue is not a great place as a night time spot to be honest. Although I could see a backpacker type really liking it. With that said it's very centrally located which is great and the reason why I recommend it.
If you are going to have him meet you at the nacional be clear where in the nacional. (outside the gates, by the pickup area, at the pool, at the park area with a small bar (overlooking the malecon) with a small bar.
How much is the casa on a per room basis? Heres a site I recommend for booking casas. I used it in January. Osmany, the guy who runs the site is a great guy and often will pick you up at the airport for a very fair price ($20 is what I last paid in January). One of the issues with a direct casa recommendation is even if you pre-book with that person who your friend recommends if another tourist goes the day before they will not honour your reservation. That is true in 99% of cases. With that said you have the benefit of having 2 people which makes people way more likely to honour your reservation as most 2 bedroom casas will often rent to 1 person and just close the other bedroom but when they rent to two people they get double the rate.
http://www.destinohabana.com/index.html (Site I recommend, Osmany is the guy who runs it. Lots of great casas which are fairly priced.) You have the advantage with a site like that in that if the one Osmany books you in cancels the reservation last minute he has a stable of quality casas. Whereas if you just book with the recommendation of your friend, in the case they cancel the reservation last minute they will just take you to a nearby casa which may or may not be of good quality and collect a commission in doing so. The advantage of direct booking is that you can negotiate a lower price but in a short stay like yours we are talking about negligible amounts and honestly if you don't know the person who you are booking with from prior experience getting a discount thats worth it is hardly likely. Osmany made sure the casa owners knew to not let my girlfriend see my visa (as I entered the country about 2.5 weeks before I saw her which I appreciated.) Also, all the casas I've stayed in have been of good quality, never ended up in a place that made me scratch my head. I can send you pictures of the last place I stayed if you'd like.
When I've gone via recommendation I've ended up at some pretty grungy places...julio can probably attest to the place I moved into during his trip which he passed a couple nights in which was about as shitty as it gets. Great people but the rooms in Canada would go for about $2 a night not 20-25.
Get your friend to send you pictures before you book anything because different people will have very different ideas of what a nice casa is. Nothing will be at the standard you are accustomed to in the US until you get into $XXX per room range. But at $35 a night you'll get a decent place.
As to Vinales if you do go make it a day trip, maybe an overnight one. I'd be surprised if you really like it. Thats a pretty small town from what I've heard. Although that does appeal to some people.
As to changing money, change at the hotels as a last option. The rates will be the worst you will get.
Heres the order
Banco Metropolitano(or other bank honestly I forget the names) > Cadeca > Airport > Hotel (So Hotels as a last option)
If you are in need of money late one night try the Melia Cohiba, easiest way to get there on the cheap is go to linea and say "linea y paseo" when the driver temporarily stops. Tell him to drop you off at paseo and walk 3.5 blocks to the melia cohiba. That will cost 40 cents per person. Otherwise you could get a private taxi for roughly $4. Personally I like going in taxi collectivos as I'm accustomed to it and it's less about the money than principle. I'd prefer to help those in Cuba who need it rather than taxi officials who make bank off tourists.
With that said typically the rates aren't that much worse at hotels. I believe the spread will typically be about 0.01 worse than the Bank rate. With that said All Inclusive Resorts in Varadero, Santa Lucia etc, the spread can be far worse. Varadero they charge some type of tax on changing your money. I don't remember the amount. I'm not sure whether they actually do or it's a little scam they had going when I went. I was pretty adamant about being upset with it as it's by no means standard practice and never has occurred to me except in my stay at a 5* in Varadero. They quickly gave me the money they skimmed off the top although they were adamant that it's a rule in Varadero.
When you change money always count it and request a receipt. Also, count the money even after they count it infront of you. One of the biggest little tricks in Cuba is miscounting. It's especially bad when you change large amounts. I've been out $50 one time and $100 another time. (These were on transactions of a few thousand)