What's commonly restricted
Most upscale Vegas nightlife venues restrict athletic wear, flip-flops or sandals, sleeveless shirts on men, shorts, and team jerseys or overly casual streetwear — broadly, anything read as 'daytime casual' rather than a going-out outfit.
It varies by venue and by night
Enforcement isn't perfectly consistent — a packed Saturday night at a top club will be stricter than a quiet Tuesday, and door staff have real discretion. When in doubt, dress a notch smarter than you think you need to.
The safe default
Closed-toe shoes, dark jeans or slacks, and a collared shirt or smart top covers the large majority of Vegas nightlife dress codes without overdressing for a more casual venue.