If you're after vintage you won't have much luck, but MBS is stuffed with high-end ADs, Malmaison is an awesome shop and worth a look around (good stock for PP, FPJ and Rolex, amongst others), some great shops in Sincere and Hour Glass in Takashimaya mall, and most brands have a boutique or two in Singapore.
Prices are high and weak £ plus VAT would kill any purchases for you, but great for a browse.
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