FTS and tri-grams

Some years ago we implemented FTS. At the time, we also added a column with the tsvector data type. We assumed we
neededthat for FTS to work. 

Later, we added tri-gram support. At the time, we assumed tri-gram complimented FTS.

More recently, we have run into a problem with replicating the tsvector column.

We now wonder if tri-gram alone can achieve a fast full text search result. If so, then we could the tsvector column be
dropped?

1. Is FTS required for tri-gram to work?
2. Are these independent of each other?
3. Is tri-gram alone sufficient for a “full text search” feature?

Any thoughts, experiences or other thoughts greatly appreciated.

 - Mark Phillips