"vom Hundert" (v.H.) means that you are adding a certain percentage of a value to that value to get a new total.
100€ + 10% * 100€ = 110€
"im Hundert" (i.H.) means that you want to add something to a value so that the amount that you added is a certain percentage of the new total.
100€ + 10% * 111,11€ = 111,11€
I would translate "im Hundert" as "reverse percentage".
We call this type of question Reverse Percentages for the amount in the question is the new amount after a particular decrease or increase and you need to find the original amount.
I don't think "vom Hundert" needs to be specified, as it's the normal way of adding a percentage of a value to that value. If you really want a translation, though, I would just call it "percentage".