I personally don’t conclude that a rootstock is vigorous until it’s grown for about a year or two and is growing really well and can survive at least one winter. With new sprouts, it’s difficult to say that it’s vigorous. Just about all Monoembryonic seedlings grow at roughly the same rate when they first sprout.
The new leaves and main stalk is fed by the seed until the first flush turns green at which point it begins photosynthesis. It is at this point where you may begin to see differences in vigor.
Polyembryonic Mangos often start out a bit smaller because the seed and thus the energy store is spread between the multiple seedlings. Even though the Polyembryonic seedlings May start out slightly smaller, the more vigorous varieties eventually catch up and may surpass Monoembryonic seedlings.
Simon