Hi Anne - I've got a few minutes so I'll try and answer a couple of the questions you have asked.
Is this cut glass or press moulded? And how can I tell the difference?
It looks like pressed glass, but that's just based on me looking at your photo. I don't recognise the design offhand, but I'll see if I can find it anywhere.
How to tell the difference between cut glass and press moulded? Pressed glass will have mould lines (where the parts of the mould come together). Even with good fire polishing (to smooth them and hopefully remove them) you can generally find a seam somewhere. Look also for characteristics of pressed glass, such as little extra "tags" of glass that might have extruded along a seam where the mould joints haven't made a tight fit. Or shear marks (a straight line) where the gob of glass was cut before it fell into the mould. There are lots of other features of pressed glass, but without diagrams and photos it gets tricky.
In my experience, cut glass has finer, sharper edges and more of a sparkle - but if the pressed glass is good quality, it can be challenging to tell.
The yellowish tinge may be caused by too much arsenic in the soda lime glass batch - I think this is what Adam Dodds explained in the past anyhow. I am sure that if Adam gets the chance to reply to your question he will give you a much better answer than I can.
Re. the black base......well my understanding of a plinth is that it raises the item (vase, bowl etc) up to showcase it. Your black base seems to do the opposite, or am I seeing things? (It's probably my perception that's up the wall!) What would happen if you turned the base the other way around? Would the bowl still fit?
I would call the glass "black". Vitro-porcelain was Sowerby's name for their glass that imitated the appearance of porcelain (ceramic).
All the above is just my opinion(s). I'm sure you'll get lots more (better) replies. :lol: