First off, I'm not familiar with this particular boiler. I'm in the USA. I believe this is manufactured in the UK and may not be distributed here.
Second, and are you sure you needed to replace the blower in the first place?
Those questions really need to be answered before making any accurate remote diagnostics. This is a pretty complex piece of equipment with costly replacement parts. Some diagnostic and/or service work should be left up to a professional in your area if you're really not sure of what you're doing.
We all (or most of us) like to save money by doing our own repairs. If you feel confident about working on this further you can go to www.vaillant.co.uk
to get manuals and service info online. This info will do you much more good than my guesses will. This unit has a diagnostic/control board that will display most faults if you know how to use it. I looked at some of the info online. They have pretty good stuff available to you if you know how to use it.
After all saying that, it "could" be the pressure switch not closing after the fan starts. You didn't say whether the new fan is starting or not. On a call for heat (or hot water) the fan starts (controlled by the main control board), the pressure switch proves the fan is running (and venting combustion gasses out of your home), it most likely has to prove water flow. If all the proving is confirmed, the ignition sequence will begin.....ignition, prove ignition, main gas valve opens, heats water, so on and so forth......
Now, if you still feel confident to work on this boiler, go to Vaillant's web site and read the manual(s). If you're not confident, call the heating man in your area. Just be careful with gas, electricity and hot water!!
I wish I could have given you more info to actually fix your problem, but I need more info as to why you changed the combustion blower to begin with. What WAS it doing? what IS it doing NOW and so on....
Good Luck to you! Let me know what you find out.