do I lose my vault items if it expires

Nope! They will wait patiently in there until you buy the vault again

ok what happen is I have 50 items in vault when it expires and only buy 1 vault slot what 25 items will I get?

However many items you have in vault will dictate how many slots you have to buy at once to activate your vault again. If you have 26 - 50 items in it when it expires, you will need to buy 50 vault slots to activate your vault again, so there is no chance of you only ending up with a 25 slot vault

There is a global setting that if turned on and you met the criteria you'd lose the items in your vault. This has never been turned on. Your ladder slasher account also meets the criteria to be exempt from the next purge.

In years past there was a rumor that if you left items in an expired vault for a year or longer they would be deleted but that also isn't true.

this rumor came from the FAQ hehe
shame on you Paul!

Yeah as you can tell the portal is out of date to say the least. Some things were added in the beginning with intent to be put in place but then never was but wasn't removed from the portal. Maps have been out a while now but still waiting on the link to be added. There's a lot of examples.