Hi Alexander,
I'm not sure about VB.net.. but I think you should be able to use something like:
CMSContext.CurrentDocument.GetValue("someFieldName", "your custom default value")
This is if the value is present in the current document. Where is your code executing? I asume you're using the ASPX model?
Might also want to have a look at some documentation:
https://docs.kentico.com/display/K9Tutorial/Creating+pages+using+ASPX+templates
Or have a look at the API samples:
https://docs.kentico.com/display/API9/Pages
Greets,
David