The Serial instance uses pins 0 and 1. DO NOT DIDDLE WITH THEM.
float inputReading = analogRead(1);
The analogRead function does not return a float.
float voltage = (inputReading);
The parentheses are unnecessary. The white space at the end is unnecessary. In fact, the whole statement is unnecessary.
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
'Temporarily closes port
The port wasn't opened. Don't close something you didn't open.
'Sets loop true
looop = True
'Loops reading serial data and displaying in textbox
Do While looop = True
tbxSerial.Text = serialportused.ReadLine() + vbCrLf
An infinite loop in the Form_Load event is a really bad idea. Don't even consider it.
The VB app runs an event loop. Let that event loop get started, and let it do the infinite looping.
Actually implementing the DataReceived event would be a good idea.