The fault is probably not at the economy gauge or it's vac hose. You do however have a massive vac leak. Based on experience, I would look at the main vac source hose.

Between the intake manifold and the firewall at the approximate center of the engine you will find a small pipe with a right angle rubber fitting attaching to the vac hose. That hose leads along the top of the firewall to the U.S. driver side where it terminates in a three-way connector. One of the hoses in that connector goes to your vac gauge.

The right angle fitting on the intake manifold is in an area of high heat and oil vapor and is usually the first to deteriorate. It is hard to see and hard to reach unless you have tiny hands. The rubber connectors are available from the dealer at low cost or an assortment can be found at most big box auto part stores. Inspect all your connectors under the hood.