Hi Drew, the virtual assistant you contract with is required to manage her tax responsibility according to her country's independent contractor (IC) laws.
As an IC, she is not your employee, so you are not responsible to address any issues of withholding or tax status with or for her.
You can submit your payments to her each tax year as a business expense (if you are located in the U.S.)
I can't really speak to your question on taxation as a middle man / referral service without more information but as long as you're not receiving funds it's not your obligation to report the income other VAs / independent contractors receive. If you're receiving monies, commissions, or similar as the middle man then you'll want to provide your CPA or tax advisor with those numbers and report the income appropriately.
I'm not a tax advisor or expert, just answering based on my own experience and would recommend double checking with a tax specialist if you have any concerns about your working arrangements and the related payments you make or receive.
I hope this helps!