Val(a)IDE have already a very simple autocompletion plugin (it just list all word in the current file), I added a better parser (based on vala parser and ctags annalyse) to list the methods, properties, signals, ... of the current object.
It's a first step, this plugin still needs some improvements (speed, completion for local variables, ...) but that allow add other features like go to the definition or help tooltips.
Note: press the Control+Enter keys to show the completion popup.