Sorry, I was too lazy to read the whole comments and so I apologize, if I repeat something....
Lets compare some tooltips of different skills and afterwards conclude, if vigor really is OP.
First off, lets compare base values of some skills:
Vigor: 1141 healing over 5 seconds (1474 in the case of the self heal of resolving vigor).
Breath of life: 1277 single target healing instant cast.
Hasty prayer: 1279 aoe healing instant cast.
Illustrious healing (prolonged healing springs morph): 259 instant cast healing plus 1036 over 4 seconds.
As everybody sees is the aoe portion of vigor smaller than the cast of one hasty prayer and also smaller than the full healing amount of healing springs. Also healing springs heals the bigger amount over a shorter amount of time and can placed several times on the ground and stacks, so that actually you can get more healing per second that vigor does over its full duration. Also hasty prayer (lets just ignore its huge cost for a moment) is the much burstier heal here, heals more than 5 seconds of vigor in an instant cast, also "allows" using this skill several times in a row, resulting in huge aoe healing. For the self healing part, the resolving vigor tooltip is a bit bigger than the one of breath of life, still the heal is over 5 seconds and breath of life can be spammed, resulting in much more healing per second if spammed.
Now that was the comparison of the base tooltips, which looks quite in favor for the magicka abilities.
One of the biggest tooltips I have seen of resolving vigor is 25k selfhealing on a stamdk with major mending, heavy armor and increased healing received from volatile armor.
The average tooltip of a magplars breath of life buffed should be around 12k, so around half the value of the above mentioned vigor. This shows, that the high weapon damage of stamina characters really profits them. Still it is very easy to compare those numbers. A magplar with 12k breath of life tooltip has the same amount of selfhealing within two seconds, meanwhile that stamdk needs 5 seconds. Also the magplar probably reaches even more healing, since he has a bonus to healing low health targets.
Hasty prayer and healing springs have even more healing than breath of life (shown above in the base tooltips), which therefore makes its aoe healing also more powerful than vigors (remember that both healing springs and hasty prayer can approximately heal for 12k per second and vigor heals that big amount over 5 seconds). Also the aoe healing component of vigor is smaller than those 25k (only the self healing is that strong).
In that case, I really doubt, that the tremendous healing OP experienced in those groups solely came from vigor. The tooltips of vigor of such ballgroup builds is by far not as strong as those 25k, also is halved in cyrodiil. So I would guess, that two people together in 2 seconds would heal a group by 6-8k, meanwhile one templar healer could heal the whole group in 2 seconds for about 20k for tremendous costs using hasty prayer. More likely it is, that those healers use healing springs tough as long as there is no emergency, which also could heal for around 10k per second, when the group stacks in the circles.