"Loads of errors, and don't know where to start" does not really tell me anything so it is hard for me to point you in the right direction.
The objective is to create a simple protocol where we place data in a bracket of a header and a terminator and VB is set up to recognise the data in it's brackets. The header is a simple word, either "Sensor1," or "Sensor2," (no spaces) this is followed by the data, either "percentageHumididy" or "percentageHumididy2" and finally the terminator which is CR and produced by using the "Serial.println" instruction. Notice the difference between print and println.
So the first sensor would transmit a string like this "Sensor1,the actual data value here"CR. The comma that follows the header is used as the marker to split the string into header and data which is then placed in an array that I call txtarray. We now end up with two strings from one txtarray(0)="Sensor1" (comma has been removed) and txtarray(1)=the value of percentageHumididy.
The visual basic code I posted is set up to capture this protocol, your Arduino code needs the following modification.
void loop()
{
int sensorVal1 = analogRead(soilSensor1);
int percentageHumididy = map(sensorVal1, wet, dry, 100, 0);
Serial.print("Sensor1,"); //header
Serial.println(percentageHumididy); //data transmitted with CR
int sensorVal2 = analogRead(soilSensor2);
int percentageHumididy2 = map(sensorVal2, wet1, dry1, 100, 0);
Serial.print("Sensor2,"); //header
Serial.println(percentageHumididy2); //data transmitted with CR