I would do it as so:
In the controller, select the customers eager loading the vehicles.
$customers = Customer::with('vehicles')->get();
return view('customers')->with('customers', $customers);
In the view:
<table>
@foreach($customers as $index => $customer)
<tr>
<td rowspan="$customer->vehicles->count()">
{{ $index+1 }}
</td>
<td rowspan="$customer->vehicles->count()">
{{ $customer->fullName }}
</td>
<td rowspan="$customer->vehicles->count()">
{{ $customer->phone }}
</td>
<td> {{$customer->vehicles[0]->licensePlate }} </td>
<td> {{$customer->vehicles[0]->brandName }} </td>
<td> {{$customer->vehicles[0]->modelName }} </td>
</tr>
@for($i=1;$i<$customer->vehicles->count())
<tr>
<td> {{$customer->vehicles[$i]->licensePlate }} </td>
<td> {{$customer->vehicles[$i]->brandName }} </td>
<td> {{$customer->vehicles[$i]->modelName }} </td>
</tr>
@endfor
@endforeach
</table>
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