You don't have to even root them in water (yes, the water can get slimy if you don't change it often). As long as you keep the soil moist, just pot your cuttings with at least a couple of the brown nubs buried in the soil. Almost guaranteed to root. As for making them bushier, after the stems grow to about 15", you can just snip or break them off halfway down and pot the broken pieces. I believe the original vine should branch out if you do this.

Pothos has been a super easy, tough plant, for me. It just needs even moisture, and bright light. Do not expect it to do well with a great deal of Sun. But if I were trying to grow Mandevilla, in the Winter, I would worry about white fly. A most persistent little pest. There may be some systemic insecticide you can add, carefully following instructions, to hold the insect off. I have damaged plants by careless use of systemics. And some plants are sensitive about hours of light. Cannot say that Mandevilla would even bloom with less than 12 hours.

I hate to be a contrarian, but have you thought about trimming off the roots a bit and transferring them to some potting soil? You would have a much healthier and probably a faster growing plant. I realize some love the idea of growing a plant in water, but unless you are using a full-on hydro setup (oxygenator, nutrient solutions, etc.), I don't feel as though it is best for the plant. I mean, totally fine for rooting cuttings--no issue there. I tried a pothos in water over a long period of time, and what I ended up with was moldy looking roots, despite frequent water changes, slow growth, and gradual decline. I trimmed and put them in soil, and the plant is huge now. Just a suggestion.

Remove the old, blemished leaves as they become breeding grounds for mold/mildew and other germs. The one leaf is yellowing so it is past its prime. The other two are blemished from some kind of pathogens/injury and will only get worse (and weigh on your nerves too as they get worse). Sometimes it is hard to remove leaves due to the old myths that the plant needs them to photosynthesize which is true but the plant should have enough reserves in its roots and stems to regenerate new leaves. I often remove every damaged leaf off some plants (outdoor plants and houseplants). New leaves often appear within a couple of weeks. Sometimes I have to cut the entire plant down due to death in the stems for whatever reason and the plant will often send up new shoots from its roots.