Heated mirrors have been a feature on Europeans cars for decades. Even the Polo has had it for years. So it's extremely unlikely that there is a fault which causes the mirrors to draw SO MUCH POWER that they get so hot that they'd actually crack!
So it would just be an unlucky case of the mirror has been hit by someone or something... it happens - and it can happen without causing damage to the mirror casing.
As for replacing the mirror glass on its own, it most certainly can be done.
ECS Tuning in the USA has them for US$65, but they're currently out of stock.
I'm sure your local VW dealer's service dept can also get them so give them a call. Let us know how you go (and the price they quote).
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