Hello and welcome to the RTA forums !

As always it is impossible to predict the weather accurately so far in advance but I would recommend you check the nps website for current information up until you leave. The other problem you have is that the Grand canyon National park is too far to go for a day visit from Vegas and have quality time there and it's not going to be easy having driven from SF to LV the previous day. To be honest I would consider doing one or the other [Yellowstone or GC] depending on conditions in Yellowstone. Note that most of the park facilities would have closed for the season already. You could have a much more relaxed time that way and if Yellowstone didn't look too promising then you could do a Grand loop from Vegas and include the wonderful NP's of Zion and Bryce canyon in Utah and Monument valley. You could also consider driving the complete loop which would be more budget friendly.