Q: Would leaking exhaust manifold cause truck to have vacuum leak?

The exhaust and intake manifolds have no connection. Therefore, leaks in one do not affect the other. The only "indirect" issue that might coincidentally implicate both manifolds is if the cylinder head were so warped, or damaged, that leaks developed on "both" sides (exhaust and intake). In regard to exhaust leaks, especially on a component such as an exhaust manifold that is ahead of the passenger cabin, be sure to fix the leak promptly. Unfortunately, it is not well understood that most mortality and morbidity from carbon monoxide is due to continuous sub-acute exposures and NOT the dramatic one-off acute poisonings you read about in the paper. Long-term exposure to even "small" amounts of carbon monoxide from a manifold leak WILL damage your health. With regard to the engine, if you have an exhaust manifold leak, extra oxygen will often get to the oxygen sensor, thus throwing off the oxygen sensor output to the PCM and that can be a significant contributing factor in poor engine operation. The moral of the story is get the exhaust leak fixed right away (for the sake of your health), then the engine operating condition can be re-evaluated, and any necessary additional repairs implemented at that point. To schedule repair and obtain an instant repair quote please utilize YourMechanic’s exhaust manifold/gasket service request page. If you request that service, the responding certified mechanic will get this resolved promptly, right at your location. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.