08:09:51 [SEVERE] Exception in thread "pool-1-thread-1"
08:09:51 [SEVERE] org.apache.commons.lang.UnhandledException: Plugin Vault v1.5.
2-b33 generated an exception while executing task 17
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftAsyncTask.run(CraftAsyn
cTask.java:56)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NoSuchMethodError: org.bukkit.Server.getOnlinePlayers()Ljav
a/util/Collection;
at net.milkbowl.vault.Metrics.postPlugin(Metrics.java:387)
at net.milkbowl.vault.Metrics.access$400(Metrics.java:63)
at net.milkbowl.vault.Metrics$4.run(Metrics.java:275)
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftTask.run(CraftTask.java
:53)
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftAsyncTask.run(CraftAsyn
cTask.java:53)
... 3 more

All of my other plugins are running fine, vault is causing these errors alone

Vault 1.5 explicitly states it is for bukkit servers built on MC 1.8. Please make sure you're using a compatible Vault version for the server version you're using.