You bring up a great question and common concern among MBA applicants. On one hand, it's important to have a fairly developed career plan and vision, which makes sense since, hey, you're applying to get an MBA... you'd better know WHY you want/need the degree given what an investment it is, financially and in terms of 1-2 years of your life, right?! Plus in a competitive admissions environment, it's easier for admissions reps to accept candidates with a clear and compelling case built through their essays and application of exactly why they need the degree for a clear and well thought-out career plan.
On the other hand, it IS normal to be not entirely sure, and in fact it's very common for students to explore different avenues during the MBA through classes, projects, and internships, and even to change direction entirely, which the admissions committee knows.
In reality you'll have to find the balance between the 2 extremes, leaning towards detailing your career plan as best as you can... if you think about it, doing so is also showing you know how to set a goal and work towards it, considering the steps and process along the way, to get there (even if, worse case, you end up changing directions later on while in school).
It's common for b-school essays to ask for short and long term post-MBA goals, so it is entirely reasonable to have 2 different visions for different phases of your career. It is good to keep in mind that VC roles in particular are so difficult to get into, so if you mention that option you'll want to illustrate that you're aware of that fact and/or do some research and thinking into how you might get to that or any specific role/career step... again show that you are able to think through all of the angles and steps to reach a goal.
Hope that helps as you start planning for those essays!
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