You have many options, easiest would be vault or nvault.
With regular vault (get_vaultdata, set_vaultdata, remove_vaultdata, vaultdata_exists), it is saved in plain-text format so you could go into amxmodx\data\vault.ini and see your saved values. The only thing you will need to do is prefix your key(s) with your plugin name or some other identifier so another plugin doesn't use your data. You cannot specify an individual vault file like nvault allows.
Code:
; Don't modify!
server_language en
SomeNumber 28
ABCPluginTimeStamp 1452397473
ABCPluginStartDate 12/26/2015 22:44
ABCPluginEndDate 01/09/2016 22:44
With nvault, you can specify the vault file name so you do not need to differentiate your keys for your plugin. But, you do need to differeniate your keys if you are saving multiple types of data using the steam-id as a 'base key'. eg. XPPoints_STEAM_0:0:12345, Health_STEAM_0:0:12345. Or, you can use nvault_get/setarray from nVault Utility and save multiple data items under the same key.
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