To use Vulse with an instrument, you'll need a way to get your instrument's 1/4" output into your iPhone. For this, you'll need a short 1/4" cable (like this Monoprice cable), and an interface.
Our Recommended Interface
Other Digital Adapters
We would avoid the iRig Pro and the Apogee JAM – they don't have any audio outputs, so they rely on the phone's headphone jack. And, of course, if you have an iPhone 7 or newer, you don't have a headphone jack, so to get these to work you would need a lightning splitter and the lightning to 3.5mm adapter that comes with the phone.
However, the Line 6 Sonic Port has all the inputs and outputs you need, so it's a fine alternative to the iRig HD 2 if you prefer it. Depending on price fluctuations, it may also be cheaper than the iRig. No velcro strap, though, which is a nice feature for Vulse, and unlike the iRig we haven't specifically tested this one, so we can't guarantee its quality.
Digital interfaces are a superior choice by far, but if you're just looking for an inexpensive way to dip your feet into this world of iPhone audio apps, try an analog interface first. You will likely experience some hissing and feedback – Vulse's built-in noise gate will help with this, but even the best algorithm in the world won't sound as clear as a noise-free digital signal.
Budget Analog Options
JamStack: An all-in-one option that is a phone mount, a cable, and an amp for output!
That about covers it for our recommendations! Let us know if any of this guide is inaccurate or incomplete.