Underpitched. Is it too late to add more?

About 15 hours ago, I pitched one vial of White Labs WLP099 super high gravity yeast into 5 gallons of 1.093 wort. I didn't have time to make a starter as this was a re-pitch. I have some time today to go get another vial, but is it worth it? When are most of the esters created that are caused by underpitching? Is it as the yeast reproduce? Do I have time to ward off any of that with another vial? Would it be worth my while?

According the The Complete Joy of Homebrewing the esters are produced during yeast respiration within the first 6-8 hours. The yeast reproduce and produce carbon dioxide and flavor characteristics of esters and diacetyl. After this cycle is fermentation when alcohol is produced. I don't know what to do about your situation because I've never made a beer with such a high gravity.

Yeah, I've never brewed anything this big either. It's an imperial stout and it's starting to krausen now, so maybe I'll just leave it. If it gets stuck or something, maybe then I'll start to think about more yeast...

It doesn't sound like your underpitched if you are getting a krausen. relax, don't worry, post a cliche'...drink something.