Me Gusta Comida Vegetariana (Calle Ricardo Arias, no phone, 7:30 A.M.–8 P.M. Mon.–Sat.) is very similar to La Casa Vegetariana [1], in both (lack of) ambience and food. Portions here are US$0.60/each. Other goodies include soy milk and veggie burgers. Its name, by the way, translates to the suitably plain “I like vegetarian food.” It’s near Restaurante Costa Azul.
Links:
[1] http://www.moon.com/destinations/panama/panama-city/food/el-cangrejo/vegetarian