I used to split foot launch instinctively, but I forced myself to learn double footed because everyone else did it. Then I had to re-learn split foot... My method is something like this.
Work out which foot you want to launch from, and where you want to put it. Work out where you want to put your other foot before you launch. Work out where you want to put your launching foot for the step before you launch.
Walk up, put your feet where they need to go, and jump. Jog, putting your feet where they should go, and jump. Run, and use the correct foot placement to vault.
Repeat until it feels more comfortable.
Another method is to launch double footed, but rather than push off both feet, have one just ahead of the other. Repeat, with the feet a little spread. Repeat again, with the feet a little more spread. Repeat again, with the feet a spread further...