Trying to find a valid product market fit is a very difficult and sometimes long journey. You've started with an idea which you think is great and is going to take off like a rocket. Then when you build this monolithic thing which is true to your idea, it fails to gain traction like you hoped for.
So, let's tackle each of your "options" that you laid out in detail.
"Continue doing lean customer devleopment, but focus on one vertical alone"
Okay, it sounds like you've done some interviewing. This is great! However, how you lay out your interview greatly determines the value you get out of it. Is your product available online? If so, take a look at survey.io as a great tool to gather customer development surveys. If your product is not online, take a look at the sample survey at survey.io/survey/demo to get an idea of the questions. You'll notice that the questions are not necessarily leading. And there's the availability for customers to provide open feedback for specific questions. The intent of these customer development surveys is not to "sell" to the customer but get honest non-leading feedback. If you perform the survey/interview yourself, you may catch yourself selling, or justifying, your product. And sometimes these customers will tell you what you want to hear. Providing a clear survey such as the one I mentioned will help avoid this.
Now, if you've done an interview/survey like this, you've got to look at it clearly. I'm going to go out on a limb and show the recent statistics of my customer development survey to show examples.
First, here's a response to a solid question:
This question is great because it immediately tells me if I'm the right track with product market fit. Over 90% of my visitors state they would be very or somewhat disappointed if my product fell off the web. The majority of those being very disappointed. I'm doing something right. If you see the majority showing Not disappointed or below, then you realize you haven't created enough value in your product or you haven't make it clear enough. You've got some work to do.
Okay, this gives me a really good idea of the competitive landscape I'm facing. I can see that I'm providing enough value to woo half my visitors from any competitor. However, based on this feedback, I can see the competitive landscape for language education, specifically Japanese, is pretty big! See the speech bubble? This gives me the chance to see what open ended answers respondents gave for this question. Most likely they'll tell me what competitors they would use and WHY. This is crazy important. Do you have the ability to combat the reasons they would use these competitors? Do you have comparable features in your product? Can you excel?
What if the majority of people said they would go to competitors? Maybe the landscape is TOO rich with well known related products. This means you'll have a very tough uphill battle to gain a foothold in the marketplace. You mentioned building a new vitamin. Imagine how difficult it is to get any level of traction in that industry.
So, based on your interviewing, you should REALLY have a strong idea of how much value you are presenting to your customers already. These surveys will tell you if you are on the right track or if you have to pivot to a different path altogether.
The fact that you are asking this question leads me to believe you haven't found a strong response in either direction in your interviews or you've got to do more. (I try to re-perform my customer development survey every quarter to ensure proper direction).
stop customer development and focus on what could be the only strong signal I have gotten once and again: foreign investors wanting to buy a small biz in the USA need guidance of all sorts, among which understanding difference among localities is a must.
First off, never stop customer development. It's a constant process and should never be considered "finished". Do you have a large enough sample size to truly see that this is the only positive strong signal you've gotten? If so, what's your domain knowledge in this international customer base? Will you be able to satisfy them? How much legwork are you going to need to do? This also requires doing competitor research to ensure you aren't putting your feet into a crowded market that will require crazy ramp up.
As part of my research around the ecosystem I started the conversation with a serial entrepreneur asking for guidance. During the conversation he offered distributing a product with certain similarities, but not exactly what I have.
Ah, the savior! When you're struggling with gaining traction, you may come across someone who wants to work with you on maybe something similar. The red flag that I heard was "serial". How many active investments is he working on? What kind of role will he play? Only distribution? As an advisor? What's his relationship to influencers in the marketplace he is proposing?
If he has 10+ active investments but is playing no active role in any of them, he's shooting for what sticks. It sounds like you need help in guidance and if this person is willing to really work with you on product development and has the pull for immediate distribution channels, sounds like it's a positive idea. However, if you go down this path, do it 100% effort. Don't think about doing a MVP in this lane while struggling with customer development with your original idea. You need to put forth your entire effort. Entrepreneurship is about taking chances and then pushing that chance with all your might.
I hope my feedback was helpful. In the end, my response in summary is this:
It sounds like you haven't done enough product market fit research to determine if the idea you have presents enough clear value. If you can't explain what your product does to the lady at the supermarket in less than 1 minute in a clear and passionate way, you haven't worked on this hard enough. If you've done interviewing and realized that your original idea didn't gain traction but some small feature in your product suite was appealing to customers (international investors funding U.S. based businesses and the tools to help them out), then do more interviewing and research into that signal to really determine it's viability.