@geekie,
The raised center in the base is commonly seen in wine bottles, and is called a punt. In wine bottles, it is thought to be a carry-over from when the bottles were hand blown, it was easier to make a bottle that could stand upright with a punt, because otherwise the pontil (where it was removed from the blow pipe) could make it unstable.
If your glass vase was hand blown, then it could be there for the same reason.
Here are some explanations for punts from wikipedia; these are specific to wine bottles, so I have edited for the reasons that may be relevent for a hand-blown vase.
https://en.wikipedia.org/wiki/Wine_bottle#Punts
A punt, also known as a kick-up, is the dimple at the bottom of a wine bottle. There is no consensus explanation for its purpose[citation needed]. The more commonly cited explanations include:[2]
- It is a historical remnant from the era when wine bottles were free blown using a blowpipe and pontil. This technique leaves a punt mark on the base of the bottle; by indenting the point where the pontil is attached, this scar would not scratch the table or make the bottle unstable.
- It had the function of making the bottle less likely to topple over—a bottle designed with a flat bottom only needs a small imperfection to make it unstable—the dimple historically allowed for a larger margin of error.
- ...
- It makes the bottle easier to clean prior to filling with wine. When a stream of water is injected into the bottle and impacts the punt, it is distributed throughout the bottom of the bottle and removes residues
