Use a common CloudFormation template for CF VPC vs non-CF VPC

I have an application that's been running in one region for years with a manually-configured VPC, but recently I updated the security groups to be managed by CloudFormation. The security group template referenced the VPC ID with a parameter that was passed in:

      VpcId: !Ref VpcId

When I redeployed my application to another region, I created the VPC with CloudFormation and I modified my security group template to reference that VPC using an output variable:

      VpcId: !ImportValue
        Fn::Sub: '${VPCStackName}-VPC'

The problem now is that I can't use this security group template for my original deployment. I tried creating a dummy template that pretended to be a VPC stack:

AWSTemplateFormatVersion: "2010-09-09"
Description: "Network: VPC (dummy stack)"

Parameters:
  VPCId:
    Type: String
    Description: >
      VPC ID

Resources:
  # Templates require a resource, create a dummy one
  NullResource:
    Type: AWS::CloudFormation::WaitConditionHandle

Outputs:
  VPC:
    Description: VPC ID
    Value: !Ref VPCId
    Export:
      Name: !Sub "${AWS::StackName}-VPC"

However, when I pointed the security group stack at it, the changeset showed that all of my security groups would be replaced even though the value of the VPC ID didn't change -- only the way it was supplied.

Any other suggestions for how I can deal with this situation? I thought this idea was pretty clever, but maybe too clever for CloudFormation.