The Australian Embassy in Hanoi had invited both government and civil society representatives to the conference.
An open letter says 50 years of one-party rule have failed to form a strong nation.
UN Special Rapporteur Heiner Bielefeldt is in Vietnam to assess the religious rights situation.
A pro-government online army is launching attacks on the site.
The rig had operated there for more than two months, damaging bilateral relations.
He says mass sharing of shaving razor blades may have been a reason.