Through Friday, 12.3% of registered Republicans had returned they ballots, while only 10.0% of Democrats and 6.9% of other. The ratio of Republican to Democrat ballots is 1.23 to 1, only slightly less than the 1.26 to 1 in 2012. There are more registered Democrats than there were then and less registered Republicans. Thus, the VBM returns are 44%D/36%R this year, compared with 44%D/38%R in 2012. This should mean that Democrats will do slightly better than they did in 2012. The caveat to that is that the biggest increase in ballots is coming from DTS and other parties. There's no reason to think those ballots won't slightly favor Republicans. There's no reason to think they will either.
To determine what the primary vote share will be by party, you want to take the 2012 vote shares and then adjust based on turnout change from 2012 to 2014, change in incumbency, and candidate quality. Ami Bera should pick up a few points for being an incumbent, while Lois Capps should pick up a few because her opponents are a lower quality in 2014. Alan Lowenthal should plus up from both of these. The data for important congressional and assembly districts is here. My predictions are here and here.