Hmm. In America, if you refer to water in a "glass" people will normally understand you to mean a container made of glass. Drinking containers made of other materials are typically called "cups", or if they are tall, "tumblers".
Of course if you are at a friend's house and you ask for a "glass of water", he may give you the water in a plastic tumbler. But I don't think that that's really because he thinks of the plastic tumbler as a "glass", but just because he thinks this is an acceptable substitution. Like if you said, "Do you have another chair?", he might bring a stool or a bench if that's all he has.
When someone wants to make clear that they mean a real-live glass and not a metal or ceramic or plastic or whatever drinking vessel, it's not uncommon in America to say "a glass glass" or "a real glass".
Of course you specified "India", so this answer may be entirely irrelevant. :-(
@Stangdon's comment is, I think, the core of the right answer.
"Skin" in English can be either a countable or a non-countable noun.
As a countable noun, "one skin", "two skins", etc, it refers to the hide of an animal, after the animal has been killed and the hide removed from the body. Like, "He kept two bear skins hanging on the wall as hunting trophies." (You could talk about the skin of a human being, of course, if you killed someone and made a rug out of his skin. But that's getting kind of creepy.)
As a non-countable noun, "skin" refers to the thing in general, of a creature living or dead. You can say, "Bob has white skin." You wouldn't say, "Bob has A white skin", just "has white skin". It's not countable.
"Vietnamese girls like white skin" is ambiguous without context. It could mean that they like their own skin to be white, or that they like men with white skin. I'm guessing you mean the second. If so, you could also say, "Vietnamese girls like men with white skin". Note that "men" here is plural, because "man" is a countable noun. ("Man" can also be uncountable, if you are talking about the human race, like "Man has written history going back several thousand years." But here we're talking about individual male people.) So "men" is plural, but "skin" is uncountable, neither singular nor plural.
Best Answer
I wouldn't use "white glass"; just look it up on Google, that means something made of glass but with a white tint, and you can't see through that kind of glass.
The glass in a household mirror is transparent, but since all glass in those kind of mirrors is transparent, you don't need to mention it. In fact, they all contain glass, so even saying "glass mirror" would be a little redundant.
If you do feel the need of describing it, "clear glass" (thanks @MichaelHarvey), "transparent glass" and "colourless/colorless glass" (depending on whether you write British or American English) are technically correct and should be understandable. The first option is used most often:
(source: Google Ngrams)