|I have had a miniature orange tree for about 10 years that I got from a friend who moved away. It was in a tapered plastic pot 13
Your citrus is probably a kumquat rather than an orange based on your description of it.
I'm not sure what you were seeing regarding the little sacs on roots as that is not a normal citrus root structure. Your plant may have had nematodes which are a pest or maybe those things were not part of the citrus roots but another growth of some sort.
Anyway, for now the best thing for your tree is plenty of sunlight, moderate fertilizing, and moderately moist soil. Sun is needed for the foliage to produce energy to support growth. The Vigoro you mentioned is fine but only sprinkle a teaspoon or so in the pot once every 2-4 weeks and then water it in well. Soluble plant food products such as Miracle Gro, Peter's, Schultz's etc. are also fine and the label provides instructions as to how to apply them. Keep the soil moist but avoid soggy conditions as these are deadly to roots and bring on root rots.
Good luck with your citrus. Thanks for the question. Please stop in again soon!