Step 1: Rinse the lemons. Cut each in half and squeeze out the juice with a citrus press. Place the lemon juice in a large nonreactive mixing bowl. Cut the rind of one lemon into 1/4-inch dice and add it to the juice. Add the soy sauce, vinegar, oil, onion, garlic, bay leaves, coriander seed, and pepper and whisk to mix. Set aside half of the lemon juice mixture to use as a sauce.
Step 2: Spread half of the remaining lemon juice mixture in the bottom of a nonreactive baking dish just large enough to hold the meat. Place the meat on top and spread the other half of the lemon juice mixture over it. Let marinate, covered and refrigerated, for at least 8 hours.
Step 3: When ready to cook, drain the meat, scraping off most of the marinade with a rubber spatula.
Step 4: Set up the grill for direct grilling using a two-zone fire. When ready to cook, brush and oil the grill grate. Place the roast over the medium-high zone of the grill and grill until cooked to taste, about 10 minutes per side for medium-rare. If the meat starts browning too quickly, move it to the medium zone. To test for doneness, insert an instant-read meat thermometer through the side of the tri-tip. The internal temperature should be 140 to 145°F for medium-rare.
Step 5: Transfer the meat to a cutting board or platter or plates and let rest for 5 minutes, then thinly slice it crosswise across the grain. Spoon the reserved sauce over the roast and serve at once with the Pico de Gallo.