I've fired my vases to vitrification so why is moisture still seeping through them?

In the vitrification process the free silica and feldspathoids in the clay melt and flow into the gaps between the clay particles, welding them together.  There are still some remaining gaps between the clay particles so the pot is not completely vitrified, if it was it would be of glassy appearance and would lack strength.  Therefore, pots described as vitreous will still have some porosity, usually this is 1% or less; in some situations, this can cause pots to leak. Added materials such as grog, silica etc are unaffected by the firing in terms of their porosity, so it is easy to understand why unglazed clay vessels will ‘seep’ when holding water. The solution is to glaze the insides of pottery that is designed to hold water with a well-fitting glaze, with no defects such as pinholing or crazing which would allow moisture to penetrate.