I previously used Zirtual for a few months, and really loved it (but my cash flow was very limited so I had to cancel it). The company has since imploded, so I'm hesitant to hire them again. Do you have any recommendations for alternatives? I am looking for US-based, and higher quality (not just someone to book meetings but also someone who can do research, create spreadsheets, create letters based on a template but that need to be customized, etc). Any recommendations?
I've worked with Virtual Assistants for about 7 years now and am a big fan of Fancy Hands. Fancy Hands has become my go-to after working with a number of virtual assistants I've found on Elance/Odesk (before they became Upwork) and Zirtual.
Fancy Hands has completed over 1200 tasks for me. Of the 1200 tasks, many have been of the type you describe, so I think you'd be very happy with their service.
They also have a tiered pricing structure, so you can find a plan that fits well with your budget. Of all the VA's I've tried, I've also found that Fancy Hands has the most favorable price/value ratio, when you break their pricing down at a cost/task level.
You can find less expensive VA's on Upwork, however my experience has been that they actually become more expensive, by either taking longer to get something right or by taking more of your time to explain something to them, which starts to defeat the purpose of using VA's in the first place!
To me, the big benefit of a service like Zirtual, that you won't get from Fancy Hands, is the historical knowledge and relationship that you build with a dedicated or semi-dedicated VA, like Zirtual gives you. That said, for the one-off types of tasks you describe in your question, I still think Fancy Hands would be a pretty solid fit for you.
Good luck with your choice and always happy to discuss further on a call!