A week or two ago, MAMP released version 3 of their all-in-one Apache server for Mac. In addition to copying a few VirtualHostX features, they've made a change to the way Apache is stopped and started. I'm still narrowing down exactly what the change is, but the side-effect is that VirtualHostX 4 and 5 will crash when they start or restart MAMP. There are a couple solutions I'm working on, but both will take a little while to implement, test, and release as I want to make sure things are done the right way. As a workaround, I've released version 5.0.4 which disables starting MAMP from within VirtualHostX. For MAMP users, all this means is that after saving your changes in VirtualHostX, you'll need to switch over to MAMP's control panel and restart Apache there. I relalize it's an extra and annoying step, but I hope you'll bear with me while I work on a new release that works around MAMP's changes.
All that said, if you haven't upgraded to MAMP 3 and are still running version 2, things should continue to work normally for you.
Further, and I've said this a thousand times in the past, if you've got a wee bit of technical chops, there's really no reason to use MAMP at all. Your Mac comes with a version of Apache built-in that VirtualHostX works great with. To that, all you need to add is MySQL. MySQL has a really nice, easy to use installer with no need to get your hands dirty in the command line, which you can download from here. Most recent Macs should download the 10.7, 64bit, DMG archive.
Update 3/30/2014: After doing some more digging, it turns out the issue is a bug (I assume a bug) introduced in OS X 10.9.2. I've been able to write a test case that exhibits the issue outside of VirtualHostX. I'm looking for a workaround and have also filed a radar with Apple. Hopefully it'll be fixed in 10.9.3, or I'll get a fix out first.