I am in Canada and I hold B1/B2 valid visa now. I will to to USA to attend a conference on the 20th of April. Meanwhile, my H1-B petition has been approved and I'll go to US consulate in Vancouver to apply my H1 visa at the end of April. My concern is that whether or not I can enter USA with my B1/B2 visa to attend the conference? I asked the same question two days ago and am still confused now. Thank you for your great help!