Like the Ledipasvir in Harvoni, the Velpatasvir in Epcluse is very insoluble. It is more soluble in an acid environment thus the recommendation about any form of anti-acid when taking either. Any sort of antacid, be it liquid antacid, chewable tablet antacid, H2 antagonists like Ranitidine or Proton Pump Inhibitors like Omeprazole, Esomeprazole, Pantoprazole, any_other_name-azole can potentially cause treatment failure due to not absorbing Velpatasvir properly.
A dose of 20mg of omeprazole should be fine (according to the product insert), but ideally take less omeprazole (or other antacids).
If you take omeprazole in the morning one strategy is to use a light dinner (food buffers acid) and your bodies natural gastric acid rhythms.
So from this you can see acid production peaks at around 11pm so if you take the Epclusa/Velpanat/VelaSof last thing before bed to take advantage of that, with the omeprazole in the morning.
An alternative strategy is to just use Sofosbuvir + Daclatasvir where we don't have these sort of problems.