V/S in the climb saves fuel? Sorry - only CLB or OP CLB will do. the most efficient climb speed is ECON determined by your trusty FMGC and the CI.
The only reason for selecting V/S is if you are climbing at a high rate (still early in the climb) and there is proximate traffic above you, selecting V/S and reducing it to +1500 or less should avoid a TCAS RA.
Now, the descent, on the other hand, is a different matter because following the managed path is NOT the most fuel-efficient thing to do.
The reason for the 'bus-struggle in the last 2000 ft to cruise level is usually because you have just found the tropopause and as everyone who has done proper high altitude training as some point in their career will tell you, that is indicated by a drop in engine performance as the air outside is no longer getting colder. The way round it? Select MACH, it will usually be .77 or .78 at this stage, reduce it to .75 or .76 and watch your little 'bus leap up to FL 370 or whatever your desired level is. Don't forget to push managed speed when you go into ALT*.