I want to make a local application connected to the web server using vb.net also the database in webserver is connected to a PHP website, how can I decode a password(encrypted using bcrypt of PHP) using vb.net and vice versa

You'd need to know how exactly the php method encrypts the password. It's like asking someone to translate from English to Manderin Chinese unless you can speak both fluently and know the grammatical rules etc. then you've no chance.

I suspect that the exact method used by the PHP is not going to be widely available - if it were, I'd look at another way of encrypting your password.

I think you may have to find a third party tool that works with both or create your own identical encryption /decryption routines in both languages.