Blanch beans in a pot of boiling salted water. Before water boils again, remove beans and immediately plunge in a bowl of iced water. Let rest for a few minutes; drain. (This will keep the beans green and crunchy.) Place in the refrigerator.
Immerse jicama in salted water for 5 to 10 minutes. Set aside.
Assemble the salad: Place beans on a platter. Top with drained jicama, cucumber, salted eggs, and tomatoes, making sure that all ingredients are evenly distributed. Cover and refrigerate.
Make the vinaigrette: Combine water and sugar in a saucepan. Let boil until sugar melts. Remove from heat and let cool. Add vinegar, pepper, fish sauce, soy sauce, onions, and chili (if using) to the cooled syrup; mix until well combined. Season with more soy sauce, fish sauce, and pepper, if desired.
Drizzle vinaigrette over salad right before serving.