to expand the OP and offer perspective
The "normal" vital signs as we have come to know them as I understand were compiled by looking at the numbers of thousands if not 10s of thousands of patients over time before they were published originally.
Pediatricians then looked at what is "normal" for kids.
Now geriatricians look at what is normal for the old timers.
All of this has something in common. They all look at people who are in the doctors office or the hospital. Not exactly the center for health and wellness. (how many people go to either when they feel great and nothing is wrong?)
To take this one step further in the effort to analyze data, medicine then came up with a whole list of "normal" numbers and more is added regularly. (so much so in medical school we stopped trying to memorize all of the lab numbers there were so many)
It was during a visit to a London hospital and attending a M&M meeting there that I had a realization. (I like to call it a "eureka moment") We were reviewing the case of a trauma patient, an elderly woman who was 67. When the intensivist gave his report, he lamented that even though they "normalized" all her numbers in the ICU, she still died. The we heard from pathology.
Sometime before this in my medical education, I learned that the human organism aapts its metabolism and functions for continued survial. In doing so, "normal" is altered and a new "baseline" is established.
It occured to me that the goal of intensive treatment shouldn't be to "normalize" the pt. it should be to return them to as near "baseline" as possible.
But that is where it becomes tricky. On an unconcious patient, how do you determine "baseline?" I have not come across a satisfactory answer. But I do have a hypothesis. By completing an exhaustive exam (physical, social, etc) and identifying as much about the patient as possible, a reasonable estimate can be made and treatment adjusted.
EMS plays a vital link in this. Because without EMS, if the patient cannot respond appropriately, there would be no record of what conditions the pt lived in which would fill in the social history.
Did the patient smoke? Did EMS notice signs of it?
Was there food all around?
Home in disrepair?
Did the patient set up their dwelling or appear to have habits that would provide clues?
It is commonly said that in EMS providers develop hyperacute awareness of their surroundings. Life is not "House MD" there is not a team of doctors who are professional housebreakers on their spare time. Why is this information never documented or relayed by EMS? What would happen if it was?
Even the location a pt is found can be of benefit. We all know what goes on in various sections of town.
As for how often to check vitals?
Well medicare/medicade in the US generally requires 2 sets of vitals to pay. So unless your organization is a charity, 2 sets at least would be a good idea.
On a critical patient, very often, I like the automated stuff to cycle about every 3 minutes or be constant.
On a stable patient, it depends on what is wrong with them, and what therapy were provided. I like a set a about 20-40 minutes after giving some PO meds for pain. Especially if I might need to give some more.
When I was doing ann inhouse rheumatology rotation they were done every 4 hours, but I really only wanted to know about 2 times a day. (once when I came in, and once before I left)
I think a good rule of thumb in EMS is when you first encounter the pt and then after every significant treatment of the pts chief complaint or condition.
I know there are a few olds that need to be shuttled back to the nursing homes in stretchers, but even in my short time, I've seen plenty who would do just as well plonked in a wheel chair and taxi. 
I stand corrected.