Basic stats to look for on a Off-tank are as follows:

25k Health- Off tanking is usually tanking the adds that arent heavy hitters like bosses, 25k is a good number to aim for when off tanking trials.

17k+ mana (having 17k+ mana is a number you should aim to be at or to improve upon though is a good base number)

25k+ stam (like mana having 25k+ stam is a good base number, going over would be fine but try not to go under it)

Much like the bars on the main tank the skills on the off tank dont vary for much of the trials as its not until the last boss in each trial in hardmode where things need to be swapped out.

If a skill bar is not made for a fight such as the first boss in vMaw it is because there is no need to change bars for that fight.