Don’t reuse a barrel in which wine has turned to vinegar; it’s impossible to get rid of the vinegar bacteria from the wood. Use the barrel as a planter in your yard.
It’s generally not a good idea to mix wine types in a barrel, or white and red wine. You will taste the previous wine in the subsequent wine.
Barrels need regular topping off with wine to keep them full. Since a barrel is porous, wine evaporates through the wood. Once a week for topping off works fine; some wineries top off twice a week. Keeping the humidity up in your winery cuts the loss of evaporation. Losing half a liter a month is normal.
The stave with the hole in it (“bung stave”) often cracks just at the hole as this is the weakest part of the barrel. Either replace the stave or seal the crack with melted bee's wax.
Wipe the area around the bung hole often with a sulfite solution. This is the area that gets seepage and spills, and the sulfite keeps this area from being a source of spoilage.
Five gallon barrels have a high surface to volume ratio. The wine can get too oaky relatively quickly. When using a five gallon barrel, keep the wine in for a shorter period of time, then blend it with wine from the same vintage that was not in the barrel; the key is to not let it sit too long.