On the one hand, gold vermeil is gold plated over a precious base
-- sterling silver -- and on the other hand gold-filled is a
thicker plate over a base metal.
Not quite annabel.
Gold vermeil is indeed gold plating on sterling or fine silver. But
it can be an exceedingly thin "wash", and still fit. In other words,
gold color without a whole lot of gold metal or value, or durability
in many cases. The term is loosely enough defined that there can be
quite a lot of variation. But you're safe generally if you just think
of it as sterling in terms of value.
Gold filled, on the other hand, is NOT a thicker PLATE.
electroplating generally produces thin layers, ranging up to thicker
electroplate layers it's true, but these are still very thin layers,
usually less than a thin foil in thickness.. Gold filled is a bonded
laminate, not just an electroplate. Generally, the percentage of the
weight of the item that's gold is a significant fraction, usually
either 1/20th, or 1/40th. Either way, if you cut through it and etch
it so you can see the gold layer thickness, you'll find it's quite
substantial. Thick enough to withstand some degree of polishing, for
example, without wearing through.
While I suppose I'm nitpicking about the nature of the difference,
you need to understand that it's a fairly substantial difference. Not
just precious silver with a plate, versus base metal with a thicker
plate. It's a MUCH thicker layer, with the gold being thick enough to
account for much of the value of the resulting metal stock.
Choosing between vermeil and gold fill might also be characterized
as what you want the piece to look like after a few years. Gold fill,
in many cases, will still look about the same. Some types will last
for decades (It depends a lot on the type of item. Chains, if worn,
for example, will grind through the gold layer in the areas between
links where they bear on each other, much faster than the gold will
just wear off the surface of a brooch.
With vermeil, however, depending on how thick the gold was put on,
the item might be showing areas with the sterling showing through
fairly quickly if the thing is used at all. And even if not, over
time, sometimes the gold will slowly dissipate into the silver, so
the color can become paler over time (whether this happens depends on
the methods used to gold plate the silver)
As a result, after some period of wear, the two types of material
will not just BE different, they will LOOK different too.
Hope that helps.