I looked up the technical specification for your power supply. The battery specifications are listed as to type dimensions, voltage and energy storage in ampere/hours. The next step is to find a battery provider that can supply this item or can construct a pack that will replace the original. Oftentimes theses packs are fabricated from a number of smaller cells put together in series and/or parallel sub sets to arrive at the voltage and amperage that is required.
If Buff no longer provides the battery as a package, perhas they can direct you to the supplier or fabricator. There are a few companies in my city that advertise that they can create packs for just about any specification. There might be similar services in the U.S. See what you can come up with and let me know- perhaps it can be made up here. Worst cases scenario is that a pack can be made up and hooked up extrinsically.
You can download the entire manual for your Vagabond unit at
www.paulbuff.com 8 OVERVIEW / Components and Specs
The Vagabond™ Lithium Extreme (VLX™) Portable Power System provides a
current-limited, pure sine wave power source, optimized for field powering
of 120V studio flash units that are normally AC-powered. The system arrives with the VLXBAT battery, the VLXINV inverter (with light stand mount
attached), and the VLXCHG battery charger.
the VLXBAT Vagabond™ Lithium Extreme Battery
The 25.6V, high capacity lithium iron phosphate
(LiFePO4) battery offers long life and increased
performance. The design includes a convenient
front panel connection for charging using the
supplied rapid battery charger.
Battery Ratings 25.6V, 6.2AH, 158.7 Wh, 20A max. cont. current
Voltage Full Charge: 29VDC, Cutoff: 20VDC
20A DC nominal output
Battery Lifespan 1000 - 1500 charge/discharge cycles
System Weight 6 pounds (excluding charger)
Component Weights 2.2-pound inverter / 3.8-pound battery
System Dimensions 6.5” h x 5” w 7.8” d (with stand clamp)
Inverter Dimensions 2.4” h x 5” w 7.8” d (without clamp)
Battery Dimensions 2.4” h x 5” w 7.5”