I see this is an old post, but I wanted to know the same and have found the answer.
We are using version 6.5, so don't know if this will work for older versions
If you connect directly to the host and right click on the host which you want to start first, select autostart and configure, you can change the startup delay of this vm only.
I can't find how to do this from vcenter server, but once you have changed locally, it is reflected in the autostart section on vcenter server.
If you need to see the config locally, you can right click on a column header and select which columns you want to see (autostart order and startup delay)