Enneagram I can see your point on, but MBTI is not vague at all if you study it.
E/I: okay, I lied, this dichotomy is extremely vague if you don't have a prominent preference (ENFPs for example test as introverts as often as they do extroverts)
S/N, F/T and P/J: basically, which cognitive functions do you use
Fe/Ti and Se/Ni: NFJ or STP
Fe/Ti and Si/Ne: SFJ or NTP
Fi/Te and Ne/Si: NFP or STJ
Fi/Te and Ni/Se: SFP or NTJ
let's say you're Fi/Te and Se/Ni, usually from here figuring out if you're SFP or NTJ is easy. if you are confused because you have a stronger tertiary and/or inferior function, the best way is to check what you were like as a kid. another way is to look at what you do when you have down time. if you're into planning, implementing plans, daydreaming, in-depth studying, you're probably NTJ. if you're into dancing, physical activity, seeking intense sensory stimulation or doing something with your hands while bored, you're probably SFP. at this point, a little more self reflection and understanding of the cognitive functions should make the differences between the last 3 letter combinations much more clear.