Double SHA256 giving me an incorrect value with this code VB.net
You seem to be interpreting the hex encoded data as a UTF-8 string. Instead, you should be decoding it as a byte dump.

To illustrate, decoding 12 as a UTF-8 string will give you:

0x3132


while decoding it as a hex dump will give you:

0x12


In bitcoin, all hashing it done on the byte values, which are often printed as hex strings for convenience - you must decode them back into a byte array as hex strings, not as actual strings.