For people with the latest version of Chrome, the spacebar does not seem to function when writing private messages. It is unlikely that Vbulletin will invest the resources in this version to repair that issue. Luckily, you can repair it your self in your settings.

Click on your Settings tab, and then go to General Settings.

Scroll down to Message Editor Interface in Miscellaneous Options, and then select "standard editor" instead of "enhanced editor". This should resolve your issue and still give you an array of edit function. Alternatively you can choose the "basic editor" option as well.