Hi Jerzshortstuff,
You should be booked already. Especially for an April wedding due to it being peak season for vacationers. Even if the travel dates aren't out yet, get it booked ASAP. My wedding is October 2011 and I have my venue booked. There really is no timeline. However, if you really have your heart set on a particular venue and/or want to do the destination wedding, book now. When you say vacation are you referring to Honeymoon?
I'm staying for a total of 7 days. Then we're off to our honeymoon. We love vacationing and normally go away for 2 weeks at a time. It's your choice and how flexible your time is as far as work is concerned. I'm doing the legal stuff here too. You won't have to worry about translating the marriage certificate and all the money that will be spent by the time it's all said and done. No one will know but you and you're hubby, if you choose to do it this way. You can celebrate your mexico wedding date for your wedding anniversary, as opposed to your official wedding date. Yes, you can marry a few days before.
I Hope I've answered your questions. If you have further, just ask! That's what this forum is for and all the brides on here are always willing and able to help.