It is one of those jobs you need to look at it on the whole and decide which way “you” feel more comfortable doing it. some things to consider…
I agree the vinyl running horizontally would be neater. but vertical drops would keep the panels and overlap pints constant all the way around the vehicle.
If you run the panels the length of the vehicle, handling of the vinyl will be much harder if installed on your own.
You will also have to wrap around the curved end and create a vertical join somewhere.
The “curved” end will be tricky! if the vinyl is hung vertical, you will not be able to get the paper out of the rear because the vinyl will be curved.
If the vinyl is curved vertically, then squeegeeing left to right isn’t going to happen, so you would be applying vertically also. by that, I mean squeegeeing up and down the depth as you wrap around.
Wrapping around the end “horizontal” will give you alignment issues. as it may run up/down by the time you get around the opposite side. to combat this, you could apply knifeless tape first where the join will be. get it straight and consistent. then apply the vinyl but with large overlaps. once in place, cut with the knifeless tape and repeat the lower panel.
Might be an idea to start with the curved area first?
If I go back to vertical panel installation. it can be done easier if working alone and the panels and the joins will be constant all the way around the vehicle, even on the curve. the downside is the curve will be tricky to apply up to and down the depth, but there lies your only real obstacle, and it is still easy enough done, just more time-consuming.