I have reproduced in my environment and got expected results as below:
Uisng setproperty:
setProperty(variables('var1'), 'Vehicles', union(variables('var1')['Vehicles'], variables('var2')['Vehicles']))
Design:

Output:

Codeview:
{
"definition": {
"$schema": "https://schema.management.azure.com/providers/Microsoft.Logic/schemas/2016-06-01/workflowdefinition.json#",
"actions": {
"Compose": {
"inputs": "@setProperty(variables('var1'), 'Vehicles', union(variables('var1')['Vehicles'], variables('var2')['Vehicles']))\r\n",
"runAfter": {
"Initialize_variable_2": [
"Succeeded"
]
},
"type": "Compose"
},
"Initialize_variable": {
"inputs": {
"variables": [
{
"name": "var1",
"type": "object",
"value": {
"Name": "SomeName ",
"Vehicles": [
{
"Brand": "Volkswagen",
"Color": "Blue"
},
{
"Brand": "BMW",
"Color": "Black"
}
]
}
}
]
},
"runAfter": {},
"type": "InitializeVariable"
},
"Initialize_variable_2": {
"inputs": {
"variables": [
{
"name": "var2",
"type": "object",
"value": {
"Name ": "SomeName ",
"Vehicles": [
{
"Brand": "Mercedes",
"Color": "White"
}
]
}
}
]
},
"runAfter": {
"Initialize_variable": [
"Succeeded"
]
},
"type": "InitializeVariable"
}
},
"contentVersion": "1.0.0.0",
"outputs": {},
"parameters": {},
"triggers": {
"manual": {
"inputs": {
"schema": {}
},
"kind": "Http",
"type": "Request"
}
}
},
"parameters": {}
}
Uisng union:
Design:
union(variables('var1')['Vehicles'], variables('var2')['Vehicles'])

Output:


Codeview:
{
"definition": {
"$schema": "https://schema.management.azure.com/providers/Microsoft.Logic/schemas/2016-06-01/workflowdefinition.json#",
"actions": {
"Compose": {
"inputs": "@union(variables('var1')['Vehicles'], variables('var2')['Vehicles'])\r\n",
"runAfter": {
"Initialize_variable_2": [
"Succeeded"
]
},
"type": "Compose"
},
"Compose_2": {
"inputs": {
"Name ": "SomeName ",
"Vehicles": "@outputs('Compose')"
},
"runAfter": {
"Compose": [
"Succeeded"
]
},
"type": "Compose"
},
"Initialize_variable": {
"inputs": {
"variables": [
{
"name": "var1",
"type": "object",
"value": {
"Name": "SomeName ",
"Vehicles": [
{
"Brand": "Volkswagen",
"Color": "Blue"
},
{
"Brand": "BMW",
"Color": "Black"
}
]
}
}
]
},
"runAfter": {},
"type": "InitializeVariable"
},
"Initialize_variable_2": {
"inputs": {
"variables": [
{
"name": "var2",
"type": "object",
"value": {
"Name ": "SomeName ",
"Vehicles": [
{
"Brand": "Mercedes",
"Color": "White"
}
]
}
}
]
},
"runAfter": {
"Initialize_variable": [
"Succeeded"
]
},
"type": "InitializeVariable"
}
},
"contentVersion": "1.0.0.0",
"outputs": {},
"parameters": {},
"triggers": {
"manual": {
"inputs": {
"schema": {}
},
"kind": "Http",
"type": "Request"
}
}
},
"parameters": {}
}
Try to replicate the same and you will get the output as i have got.