The snmp_vals slot is a LIST_OF STRING data type. That means it is a comma-separated list of string values. In MRL, lists of strings behave differently than simple strings. You can extract individual values, one at a time, from a LIST_OF STRING, using the MRL listgetelt function. You extract each value based on its numeric sequence in the list. The first item in the list is element 1. See the documentation I link to above for an example of the syntax.
You can also convert the entire snmp_vals list into a regular string with the listtostr function. However, this actually makes it harder to work with for most purposes. I recommend you use listgetelt because it allows you to handle each snmp_vals element individually.