Not sure if you are talking about vending tri-tip sandwiches, or sliced for dinner. For sandwiches here is how I do it. (on a cookshack 250)
Rub them with spices, and set overnight.
Smoke them to 145 degrees internal. Pull them and cool as quickly as possible. Local regs. here require 2 inches max. thickness when cooling, tri-tips qualify for this.
When cool, I slice and portion to 5oz. portions (for sandwiches)wrapped into foil. If you have a slicer, slice them paper thin. Save the juice left in the cooling pan, it has all the flavor from the rub. These can be kept refrigerated or even vac. bagged and frozen if left over after event.
When vending I use my flat grill to heat. Throw a pre-measured portion onto the grill, "sizzle" it with a squirt of the saved meat juice, cover with a lid for a few seconds. During this time I am also toasting the buttered hoagie roll on the grill. This all takes less than a minute and you have a hot toasted roll, with juicy meat up to the required 165 degrees and the customers love the "sizzle factor" of a freshly made sandwich. I usually have something smoking in the cookshack at the same time to add to the smoke ambiance.
For dinner I also smoke to 145 degrees. (usually remove at 140 and cover in a pan or pot to rest and rise to 145) After resting at least 30 minutes you will have lots of juice in the pot. Then remove tri ti tips and slice to dinner thickness, usually 1/4 inch. the meat will still be pink in color. I hold the sliced meat in full size pans in a pre-heated cambro or ice chest till ready to serve. Bring the reserved juice to a boil and hold it for serving time. Pour over the pans of meat just prior to serving. It will take the pink out which some may find worrisome. If holding for more than an hour or so, I would add some of the juice to the foiled pans after slicing to keep them warm. You just don't want a whole bunch of it too soon or you end up with steamed meat which isn't as appetizing imho. Hope this helps
Jack