I found this in the console:

[Vault] Task #2 for Vault v1.5.6-b49 generated an exception java.lang.NoClassDefFoundError: net/milkbowl/vault/Vault$1$1 at net.milkbowl.vault.Vault$1.run(Vault.java:159) ~[?:?] at org.bukkit.craftbukkit.v1_10_R1.scheduler.CraftTask.run(CraftTask.java:71) ~[server.jar:git-Spigot-6016ac7-10c10b3] at org.bukkit.craftbukkit.v1_10_R1.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:350) [server.jar:git-Spigot-6016ac7-10c10b3] at net.minecraft.server.v1_10_R1.MinecraftServer.D(MinecraftServer.java:741) [server.jar:git-Spigot-6016ac7-10c10b3] at net.minecraft.server.v1_10_R1.DedicatedServer.D(DedicatedServer.java:399) [server.jar:git-Spigot-6016ac7-10c10b3] at net.minecraft.server.v1_10_R1.MinecraftServer.C(MinecraftServer.java:672) [server.jar:git-Spigot-6016ac7-10c10b3] at net.minecraft.server.v1_10_R1.MinecraftServer.run(MinecraftServer.java:571) [server.jar:git-Spigot-6016ac7-10c10b3] at java.lang.Thread.run(Thread.java:745) [?:1.8.0_74] Caused by: java.lang.ClassNotFoundException: net.milkbowl.vault.Vault$1$1 at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[?:1.8.0_74] at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:101) ~[server.jar:git-Spigot-6016ac7-10c10b3] at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:86) ~[server.jar:git-Spigot-6016ac7-10c10b3] at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_74] at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_74] ... 8 more

Latest version

Are you trying to load Vault multiple times or have a bad download of it? It's saying it can't find the Vault main class, or there might be another plugin trying to reload vault even though it's already loaded. either way, this isn't a bug in Vault itself.