Table design problem
If your vad_vaf_id does not sufficiently designate the target value, perhaps you should add a new column in your temporary table to provide that information? Something like vad_target?

+-----------------+   +-------------------+
| validation_data |   | validation_fields |
+-----------------+   +-------------------+
| vad_id          |   | vaf_id            |
| vad_met_id      |   | vaf_field         |
| vad_vaf_id      |   +-------------------+
| vad_target      |
| vad_value       |
| vad_is_treated  |
+-----------------+


When user changes his firstname, leave the field NULL. When there are multiple possible targets (i.e. with inverters), use inverter's id.