The ‘normal’ method is to use oneBLYNK_WRITE(vPin) callback per virtual pin in use. This then runs code which is explicit to that particular virtual pin, so negates the need for all of the if/switch case code that is necessary with the BLYNK_WRITE_DEFAULT callback.
The potential drawback to the ‘normal’ approach is that if you have multiple widgets that do very similar things then there can be quite a lot of code duplication within the various BLYNK_WRITE(vPin) callbacks. This can usually be negated by calling a function from within the BLYNK_WRITE(vPin) callback, so the only time that I really use BLYNK_WRITE_DEFAULT is is very specific and specialised situations such as this one…
The normal approach would be to use serial print messages.
It’s impossible to say when you just post snippets of code instead of the full sketch.
Pete.