Chazal tell us that vayehi tends to introduce a bad event ; tzaar (gemara megillah 10b; see Rabbeinu Chananel there). For example, that’s how the megilla starts off. However, ‘vehaya’ is an expression of simcha (as in the opening of this week’s sedra). Why do these two words imply these 2 very different things? Each word is made up of a word and a Vav. For example, ‘haya’ alone (with no Vav) means ‘it was.’ The vav changes it to mean ‘and it will be’ (vehaya; future). Whilst ‘yehi’ means ‘it will be’ and the vav changes it to mean ‘and it was.’ A vav itself is a letter of connection between higherand lower world (it means ‘and’ – a connector).
Now, we know that the future for the Jews will be bright; we are promised survival and the ultimate arrival of Moshiach. But we also know that there have been pains along the way; especially over the last century. The question is does one use those past pains to define the future, or does one use that future to change one’s perspective of those past pains.
In other words, one can either say ‘it has been so painful, I do not believe that the future is really going to be good’ or one can say ‘I know that we’ve been promised that the future will be good, and thus I am realising that the past pains are necessary.’ The former approach is nothing but tzaar whilst the latter brings simchah. And this is the difference between the 2 words vehaya and vayehi. Vayehi denotes tzaar for it changes the future (yehi) into the past (vayehi) ie it uses the past to redefine the future perspective, whilst vehaya denotes simchah for it uses the future (vehaya) to redefine the past (haya) via that connection (the vav).
