I tend to agree with you that outsourcing projects under your company to someone you don't know, and for a skill you have no way of providing should something go wrong, is a slippery slope and you're wise to take another approach. As you get to know more VAs and other service providers, you'll likely come to know people you feel comfortable sending your clients to should they request this or any service you don't provide.
As to how to best work with a developer I'd say that applying your knowledge of working with clients to actually being the client will give you a good head start. Also, as a business owner, it pays to do a bit of research so you're coming to the table armed with information and at the very least a few questions that should be answered.
How much of a deposit will you need to make to get work started?
How will you update your website later? Does it come with a CMS or is that a sep. charge?
How many revisions are you going to be given during the process of designing your site? And how much will possible further changes at your request cost?
Is there a kill fee should you decide this designer isn't going to fulfill your vision for this site?
Who will pay for images, content development, is
SEO included in the cost of your site?
Does your developer code to updated standards for web development and
SEO? [this you can see by viewing the source code of a site they've done and checking the code in the W3C site, no need for 100% but it should be fairly high compliance.]
If your site design includes hand drawn or computer drawn graphics, will your designer be using stock images or will they be creating fresh designs for you? Same goes for, will they be using a template or will they create something custom for you?
If you are hosting with the designer how many emails, databases, and other features like autoresponders and basic statistics about traffic, etc. are included and at what cost? If you have to pay extra for these things then it's not a 'good deal'.
Those are just a few suggestions - hope it helps