'Attaches a VPN gateway to a VPC.
:type vpn_gateway_id: str
:param vpn_gateway_id: The ID of the vpn_gateway to attach
:type vpc_id: str
:param vpc_id: The ID of the VPC you want to attach the gateway to.
:type dry_run: bool
:param dry_run: Set to True if the operation should not actually run.
:rtype: An attachment
:return: a :class:`boto.vpc.vpngateway.Attachment`'

def attach_vpn_gateway(self, vpn_gateway_id, vpc_id, dry_run=False):

    params = {'VpnGatewayId': vpn_gateway_id, 'VpcId': vpc_id}
    if dry_run:
        params['DryRun'] = 'true'
    return self.get_object('AttachVpnGateway', params, Attachment)
