Bird's-eye chili (ớt) on phở garnish plates is intensely hot. Tear off small slivers, don't crunch a whole pepper.
Vietnamese spice tolerance is high. If you bite a whole chili, eat rice (not water) — capsaicin is oil-soluble.
If a dish is too spicy, ask for 'thêm rau' (more vegetables) and 'thêm bún' (more noodles) to dilute it.