Check your virtual assistants online reputation that you can based if he/she can be trusted. Another thing is you can also ask his/her for past referrals regarding confidentiality of the clients. Then, you can ask them how the virtual assistant handle this tasks.
Aim for Filipinos, they tend to be more reliable. Build a relationship with them, don't give away your credit card number straight away.
After a few weeks/months, you will know if you can trust them.
As a provider of Virtual Assistants myself, I think the best way to go is to ask for referrals and speak to their current clients for feedback on their services. Once you have a done that and your gut still says you should still proceed (notice, you have to listen to your gut), I suggest you start small; build the relationship gradually and enhance your trust for them over time.
On my blog I interview successful Entrepreneurs on How they make use of Virtual Assistants in their businesses to free their time to focus on why they love their businesses. Please check out my blog and hopefully, you will be able to learn from their successes and challenges with the entire process of outsourcing and making use of VAs.