Hi Carla,
No, a US company can use a non-US virtual assistant. If they ask for them, point out that you cannot fill this out because you are not a US citizen and do not have US identification numbers. Point out that you pay your taxes to your home country and are responsible for that. You do not get a 1099. I have several clients that use folks in different countries. W9s are IRS forms, you don't answer to the IRS