HH relates to the water pressure a fabric can withstand before it leaks.
So if you put a long long tube over a piece of the fly sheet of the Vango, and filled it up with water, the water would be 5000mm high (5metres) before the water would start to come through.
Likewise the outwell could support 3m of water before it started to leak.
However, if you go to ukcs and do a search on HH you will see it is quite a contraversial 'measurement' and some feel it is being used by the manufacturers as a big selling point for their tents.
iirc someone over there said that in the olden days (so like 10 years ago) most tents bought in this country (ie 2 season family camping tents) only had a HH of 1500 and they didn't leak. Now the benchmark is usually 2000...so people will advise you that your bargain from Lidl with a HH of 1000 may not withstand a typical british Bank Holiday.
With regards to your 2 tents, I don't imagine the HH will make much of a difference, as if it rains most leaks will occur through the seams anyway (if they haven't been taped correctly).
tbh if I was buying another tent (and I'm not, I'm absolutely not oh no DH would kill me), I would be looking at the designs to compare how they stand up to wind. After this BH and all the tents that have been trashed, I would be more concerned if the thing will stand up to a windy day, rather than how waterproof it is.
hth
jeez I am such a nerd.