Welcome to the Forum!
Let's start getting you some help... but first can you answer these questions for us? A little more information is needed to better assist you...
- What is a microvessel in this image? Can you tell us what they should look like? (Not all of us on here are biologists or if we are, we aren't necessarily looking the same types of samples, etc. - so be as specific as you can.)
- Adjusting Brightness & Contrast is only for rendering purposes, so this will not change your analysis results/calculations. What is the exact procedure you are currently applying?
- When you say "the subtract background isn't working...", what exactly do you mean? You are getting a 'bug' in the software or the results just are not what you want?
Also - if you can post the raw image/dataset (as opposed to this .png file) - that would be ideal.
To get more familiar with Segmentation in ImageJ/Fiji, here are some helpful links:
Hope this helps!