Units will use the technology of their province, bear in mind that 'Counts' do not generate technology points which means it is 'Dukes' and above (this will be the Duke's Capital) that will affect the ability of units in that province.
A tactic often used to help spread technology is to focus on your 'Capital' and then use your 'Spymaster' to research technology from a high-tech hub - such as Constantinople - then using your councillors to spread the pressure of technology from your Capital to neighbouring provinces / Duchies. Doing this will help both your other provinces and vassals in terms of tech and thus allowing them to achieve higher tech bonuses.
Another important note worth mentioning is that tech will spread to all neighbouring provinces, regardless of the fact they might be in a different country, in effect this means that if your capital is bordering another countries provinces then you will be in effect increasing their tech. This is why it is usually a good idea to have your capital positioned in a fairly central position within your empire.