1. - is, because 'which I vividly remember' is in the present tense. 2. I think 'was' *note, vividly remember / off the top of my head is not a good combination. vivdly remember inplies a very strong memory, off the top of my head implies you didnt have a strong memory to make your decision easy so you picked one memory at random. ** I would phrase your second sentence differently if it is on its own - 'I read an article recently about google'. Your form is ok if it is part of a paragraph like: 'I have been reading lots of articles lately. I like to read the articles on msn.com when i have a few minutes to spare. one of the articles which I have read recently was about google. I learned that they changed their logo.' 'one of' implies there are others, and the listener will be wondering why you are telling them there are others. If you already gave that context it is ok.

I would suggest using 'was' because you are referring to something that has already happened (Past tense). So for example: One of the articles which I read recently was about Google