Bottoms of glass items can give clues as to how and when or even where they were made. Those two vases aren't tat; they can come and live at my house. In the the late 19th early 20th century there were hundreds of companies involved in glass in Europe, either making it or decorating it, and even more in the US. Company records have been lost for many, many of them, and wholesalers records don't always give who, if they give where. Everyone copied everyone else, just to make things harder.
The only way to learn about glass is to read message boards like this one, buy and read specialist glass books, handle stuff at antique centres, look at stuff on the internet, taking some of it with a pinch of salt, and never believe everything you see on ebay or read on labels.
This is just some of my collection and probably half of it is ID'd but more has a time period and a country provisionally assigned to it. I'm warning you, it's an addiction