Just a quick try using sitemap (with this mapping) on the Thermostatmode item.. It works like a dream:
Selection item=VmbeloMode mappings=[SAFE="SAFE", NIGHT="NIGHT", DAY="DAY", COMFORT="COMFORT"]
But I guess what you´re trying to do is to control the acutal heater, right? Thats the OpMode item and not the ThermostatMode item.
This tells that your item changed from SAFE to heat..
BUT! thats the ThermostatMode item.. And it does not accept the “heat” command as far as I understand.. So it reject the command ofcouse.
You problem is, you´re trying to control the thermostat with commands belonging to another item..
Thats why I asked for the possible modes for the glasspanels 
But are these two modes read/writeable or only readable?
It takes only a few seconds to test using sitemaps
EDIT.
Tested the OpMode in sitemap using this mapping:
Selection item=VmbeloOpMode mappings=[HEATING="HEATING", COOLING="COOLING", OFF="OFF"]
This is the outcome.
2020-03-26 17:33:58.780 [ome.event.ItemCommandEvent] - Item 'VmbeloOpMode' received command COOLING
2020-03-26 17:33:58.797 [nt.ItemStatePredictedEvent] - VmbeloOpMode predicted to become COOLING
2020-03-26 17:33:58.832 [vent.ItemStateChangedEvent] - VmbeloOpMode changed from HEATING to COOLING
2020-03-26 17:33:59.005 [vent.ItemStateChangedEvent] - Vmbelo1TempSet changed from 22.0 °C to 21.0 °C
2020-03-26 17:33:59.355 [vent.ChannelTriggeredEvent] - velbus:vmbelo:1e8ce9e5:02:input#CH12 triggered PRESSED
2020-03-26 17:33:59.359 [vent.ChannelTriggeredEvent] - velbus:vmbelo:1e8ce9e5:02:thermostat#COOLER triggered PRESSED
2020-03-26 17:34:30.565 [ome.event.ItemCommandEvent] - Item 'VmbeloOpMode' received command OFF
2020-03-26 17:34:30.574 [nt.ItemStatePredictedEvent] - VmbeloOpMode predicted to become OFF
2020-03-26 17:34:30.599 [vent.ItemStateChangedEvent] - VmbeloOpMode changed from COOLING to OFF
2020-03-26 17:34:30.614 [vent.ItemStateChangedEvent] - VmbeloOpMode changed from OFF to COOLING
2020-03-26 17:34:30.877 [vent.ItemStateChangedEvent] - Vmbelo1Temp changed from 24.4375 °C to 24.5 °C
2020-03-26 17:34:54.052 [ome.event.ItemCommandEvent] - Item 'VmbeloOpMode' received command HEATING
2020-03-26 17:34:54.060 [nt.ItemStatePredictedEvent] - VmbeloOpMode predicted to become HEATING
2020-03-26 17:34:54.075 [vent.ItemStateChangedEvent] - VmbeloOpMode changed from COOLING to HEATING
2020-03-26 17:34:54.415 [vent.ItemStateChangedEvent] - Vmbelo1TempSet changed from 21.0 °C to 22.0 °C
2020-03-26 17:34:54.853 [vent.ChannelTriggeredEvent] - velbus:vmbelo:1e8ce9e5:02:input#CH12 triggered RELEASED
2020-03-26 17:34:54.857 [vent.ChannelTriggeredEvent] - velbus:vmbelo:1e8ce9e5:02:thermostat#COOLER triggered RELEASED
Notice.. OFF did not work. HEATING and COOLING worked fine, and it triggered a input channel.