OMD, you are very astute. As it happens, yes, the additional VCI subtests that are included in the VECI are allowable substitutions (and likewise the EGAI, etc.), but only intended for use as substitutions in the FSIQ. That is, substitutions are not allowed for any other index-level score.
As to the CIs, Pearson chose to base them on estimated true scores (using the standard error of estimation (SEE)), as a partial correction for regression to the mean. The size of the SEMs appears constant largely because they list them as integers. In the tables included in the technical manual, it is apparent that there is variation by age (you can see this in the freely-downloadable expanded indices technical report (#5), too). Although not expressly listed, it can be inferred from the tables on significant differences between measures that the SEMs also vary by ability.
I expect that the variation (across both age and ability) was judged sufficiently minor that the functional impact of simplicity (rounding to whole numbers) was considered more important than that of precision.
