You can't just start providing SEO service because you've virtual staff.
First you'll need to make sure, your virtual staff knows what is SEO and how SEO works and what precautions they should take while doing these tasks. This means you'll need to train and teach them stuff they don't know about SEO.
Alternatively, you can interview your virtual staff first and ask what specialties and work experience they have. And then create gigs based on their expertise.