swift --version
# Apple Swift version 4.0.2 (swiftlang-900.0.69.2 clang-900.0.38)
# Target: x86_64-apple-macosx10.9

brew info vapor
# vapor/tap/vapor: stable 3.1.2
# /usr/local/Cellar/vapor/3.0.3 (4 files, 16.7MB)
#  Built from source on 2017-10-17 at 11:16:19

vapor new TemplateApiTest --template=api
cd TemplateApiTest/

swift package tools-version
# 4.0.0

rm Package.resolved
vapor update

vapor --version
# Vapor Toolbox: 3.1.2
# Vapor Framework: 2.3.0

Set the Xcode schema to be "Run > My Mac".

There are about 44 new deprecation warnings of the same type at this time.

'characters' is deprecated: Please use String or Substring directly".

This project did build and run OK (0 errors) with Xcode 9.1 (9B55) + Vapor 2.3.0 + Swift 4.0.2.