IMHO you should process your whey for Ricotta immediately and not let it sit for a day or two. You may or may not have to use vinegar depending on the acidity of the whey. For example, whey from Mozzarella (quicky or traditional) is already acidic enough. Whey from a Colby is going to be over 6.0 so that would have to be acidified.
"Real" Ricotta is traditionally made from Mozzarella whey, so there would be very little or no need for additional acidity.
At a meeting of the local cheese guild, there was a conversation about ricotta making and how they whey never comes clear. One of the members observed that he always let it sit for at least 24 hours, and it always came clear. A little research on the internet found several sites that instructed to let the whey sit for 12-24 hours, so I've always felt comfortable doing that. Since I don't make Mozzarella, I rarely have whey that's acid enough, and I prefer the taste without the vinegar.
I don't think the age of the whey, though, has anything to do with the graininess, since this is something new. Of course, I've been making new cheeses lately, new for me, that is. I wonder if that's the difference?