I had no problem shooting platic vanes off a standard arrow rest, in spite of the warnings of the traditionalists.
The spine of the arrow matters more than the fletching anyhow.
If the arrow is the right weight for the bow it wiggles around the bow as it leaves and the fletching does not touch the bow.
The effect is called The Archers Paradox.
It the arrow is too stiff it will not fly straight because it slides against the bow as it leaves. If the spine is too weak the arrow bends too much and the nock end slaps the bow as it leaves.
As for old fiberglass, I have shot very old glass bows with no problems too.
Just try bending the bow easy the first few shots. If it is going to fracture it will likely start by delaminating on the back, most likely near the edges of the back. It wont be a brittle type of a fracture, more of a splintering effect. It moght separate from the wood core if it is a fiberglass backed wood bow. Before stringing it look it over to make sure there are no nicks or whitish looking areas.
As fiberglass delaminates the resin pulls away from the glass and gets a milky instead of a clear look to it.