One solution - upgrade VCD 1.5 to 5.5 (just have to do a few upgrades in the middle, ie 5.1)
That said, if the two cloud are already up and small you can use the export option, but 1.5 does not have a like for like export. You will need to setup the networking in 5.5 to be as close to 1.5 as you can first. Then export/import vapps. Once the apps are imported you will need to correct the working uplinks, and possibly MAC/IPs.
Far from ideal, but the export/import path is not really the migration path VMware seems to have planed for; the official migration path seems to be do the stepped upgrade to all the versions between. (Including VCNS, VCenter, ect) -- If you need older versions and can't find them support has been able to assist us in the past with that.