AWS CDK: Create a Subnet and Launch Instance in it With Existing VPC

I have a fairly simple task that I am unable to complete with AWS CDK: Create a subnet and launch an instance in it using an existing VPC.

The existing VPC is a CfnVpc that I obtained using the cdk migrate command. Here it is:

class Lab(Stack):

    @property
    def vpc(self) -> ec2.CfnVPC:
        return self._vpc

    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(
            scope, construct_id, description="Nat instance with two subnets", **kwargs
        )
        self._vpc = ec2.CfnVPC(
            self,
            "VPC",
            enable_dns_support=True,
            enable_dns_hostnames=True,
            cidr_block=subnetConfig["VPC"]["CIDR"],
            tags=[
                {
                    "key": "Application",
                    "value": self.stack_name,
                },
                {
                    "key": "Network",
                    "value": "PublicA",
                },
            ],
        )

Here are my new resources:

class MyResources(Lab):
    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        vpc = ec2.Vpc.from_lookup(
            self, "L2VPC", tags={"aws:cloudformation:logical-id": "VPC"}
        )

        publicSubnetB = ec2.Subnet(
            self,
            "PublicSubnetB",
            availability_zone="us-east-1b",
            cidr_block="10.0.2.0/24",
            vpc_id=vpc.vpc_id,
        )
        cdk.Tags.of(publicSubnetB).add("Application", self.stack_name)
        cdk.Tags.of(publicSubnetB).add("Network", "PublicB")

        # These are also exposed from the cdk migrate command via properties. 
        ec2.CfnSubnetRouteTableAssociation(
            self,
            "PublicSubnetBRouteTableAssociation",
            subnet_id=publicSubnetB.subnet_id,
            route_table_id=self.publicRouteTable.ref,
        )
        ec2.CfnSubnetNetworkAclAssociation(
            self,
            "PublicSubnetBNetworkAclAssociation",
            subnet_id=publicSubnetB.subnet_id,
            network_acl_id=self.publicNetworkAcl.ref,
        )

And finally, here's the failing instance:

        instanceB = ec2.Instance(
            self,
            "InstanceB",
            instance_type=ec2.InstanceType.of(
                instance_class=ec2.InstanceClass.BURSTABLE3,
                instance_size=ec2.InstanceSize.MICRO,
            ),
            machine_image=ec2.AmazonLinuxImage(
                generation=ec2.AmazonLinuxGeneration.AMAZON_LINUX_2
            ),
            vpc=vpc,
            vpc_subnets=ec2.SubnetSelection(
                subnet_type=ec2.SubnetType.PUBLIC, availability_zones=["us-east-1b"]
            ),
            user_data=ec2.UserData.custom(userDataB),
            private_ip_address="10.0.2.119",
            associate_public_ip_address=True,
            security_group=sg,
            user_data_causes_replacement=True,
        )

AWS CDK says that there is no public subnet in az us-east-1b - even though I created it just prior. The only way this works is if I comment out the instance code, run it, clear the context, run it again, and comment back in the instance code. There has got to be a better way!