I am aware that a Warforged Envoy picking anything but the tools in the PHB on page 154 is probably against RAI and would be an awkward implementation. But is it RAW?
The Warforged Envoy's Integrated Tool (WGtE, p. 69) feature says:
Choose a tool you're proficient with. It becomes integrated into your body, and you double your proficiency bonus for any ability checks you make with it. You must have your hands free to use this tool.
Backgrounds such as Sailor and Soldier include "vehicles (land)" and "vehicles (water)" as "Tool Proficiencies" (PHB, p. 139-40). Furthermore, Xanathar's Guide to Everything includes "Land and Water Vehicles" under the section titled "Tool Proficiencies". It is organized alphabetically between "Jeweler's Tools" and "Leatherworker's Tools" (XGtE, p. 82).
Then it seems that vehicles are tools that you can have a proficiency with, and therefore can have integrated in your warforged envoy.
Is my understanding correct?