Q:

Create a VM in a resource group using my own vhd

In the classic portal/ASM I could use some simple PowerShell cmdlets to create a VM using my own vhd. Once the variables were set the flow was pretty much Add-AzureVhd > Add-AzureDisk > New-AzureVm.
For the life of me I cannot find any documentation on creating an ARM VM using PowerShell with my own vhd.
Can anyone point me in the right direction?
Edit: Here is the code I am using.
$rgName = "somerg"
$location = "centralus"
$storageName = "somestorage"
$storageType = "Standard_LRS"

$nicname = "client1nic"
$subnet1Name = "Subnet-1"
$vnetName = "somevnet"
$vnetAddressPrefix = "10.0.0.0/16"
$vnetSubnetAddressPrefix = "10.0.0.0/24"

$vmName = "Client1"
$vmSize = "Standard_A2"
$osDiskName = $vmName + "osDisk"

$pip = New-AzureRmPublicIpAddress -Name $nicname -ResourceGroupName $rgName                 -Location $location -AllocationMethod Dynamic
$nic = New-AzureRmNetworkInterface -Name $nicname -ResourceGroupName $rgName    -Location $location -SubnetId $vnet.Subnets[0].Id -PublicIpAddressId $pip.Id
$vm = New-AzureRmVMConfig -VMName $vmName -VMSize $vmSize$vm = Add-  AzureRmVMNetworkInterface -VM $vm -Id $nic.Id
$osDiskUri = "https://somestorage.blob.core.windows.net/vhds/Client1.vhd"
$vm = Set-AzureRmVMOSDisk -VM $vm -Name $osDiskName -VhdUri $osDiskUri -  CreateOption attach -Windows

New-AzureRmVM -ResourceGroupName $rgName -Location $location -VM $vm -  Verbose -Debug

A:

Set-AzureRmVMOSDisk has three options when creating an OS disk using the -CreateOptions The first is fromImage which creates an a machine from a prior image. 
There is then attach which attaches a preexisting disk, which would be the option you require. 
There is also empty but I've not found the use case for that as yet!
Anyway, this should do what you need. 
Set-AzureRmVMOSDisk -VM $vm -Name "test" -VhdUri $uri -CreateOption attach -Windows 

Wrapped into the rest of a VM Create script 

