回複：急問去加拿大開會回美國要美國簽證嗎？

If you have valid I-94, you don't need VISA to enter USA. But you need VISA to enter Canada. Keep your I-94 when you leave USA, just tell them you will come back USA within 30 days. When you come back, show your passport with valid I-94. And if you go to Vancouver, the USA customs locates in Canada instead of in USA.