This is an actual problem. I'll say the same thing in a different way than other contributors. Also, I can see why students have difficulty determining actual vs potential problems. I guess the a good way to determine the difference is to look at the problem and determine if you have evidence (assessments) in the present moment or do you foresee a problem occurring?
With that being said, we have evidence in the present moment. Therefore, this would be an actual problem. Here is your evidence:
1. Vomit 4x in 12 hours
2. No food or liquids
(No appetite or ability to take in fluid, hence the necessity for IV fluid)
3. BP 90/50
(Loss of intravascular fluid leading to decreased BP)
4. RR 28
(Could be compensatory to related to the fluid loss to deliver oxygen to tissues)
5. HR 110
(Compensatory, when BP goes down, HR usually goes up)
6. T 102.7
(Sign of shock coming, the elevation maybe related to warmth loss from fluid loss)
(At risk for fluid loss related to being 85, normal loss of thirst mechanism, thinner skin, and the possibility of heart of renal problems)
As far as problem priorities go, here's how I arrange them. First of all, in this scenario, there is one main problem. Fluid loss for some unknown reason. Vomit is potentially the cause but nonetheless we don't know the cause. But, I prioritize problems by what is going to kill the patient first. Morbid, I know. But, it does work. Fluid loss here would kill the patient first. Fluid loss would lead to decreased cardiac output leading to organ failure (heart, kidneys, etc.) and hypovolemic shock then likely death or permanent organ damage.
Needless to say, in this case, hypothetically, if there wasn't the evidence above, you realize that there is a potential fluid loss problem. What is the difference? Either way, actual or potential, you are addressing the problem.
I hope I haven't confused you more.
Hope this helps!