First of all I don't have a powermeter however I have read THE book.
I would think that if you have been on vacation and not ridden then you dont seed, just treat it as a break.
From my understanding, seeding is used for initial setup to give the software a starting point. Therefore if by seeding you want to enter training loads for the time you have been off then the info both in and out of the software will not be accurate. I think seeding is for when you have been training but not recording values or have lost values etc.
From Training Peaks:
"3) Related to the above, the long time constant for CTL means that data must be collected for a fairly long period of time before the Performance Manager calculations can be considered accurate (cf. Fig. 3). Obviously, however, a new powermeter user will not have a large database of files that can be analyzed to determine their starting point. Similarly, a long-time powermeter user who hasn’t paid sufficient attention to tracking changes in their functional threshold power may not wish to rely on their previous data, or they may be without their powermeter for a lengthy period of time (e.g., while it is being repaired). In such cases, it may be necessary to “seed” the model with starting values for CTL and ATL, by using the “Customize this chart” option for the Performance Manager chart in WKO+. The appropriate value to use can be estimated by realizing that most people train at an intensity resulting in 50-75 TSS/h (i.e., average weekly IF is usually between ~0.70 and ~0.85). Those who train more, mostly or entirely outdoors, and/or in a less structured fashion would likely fall towards the lower end of this range, whereas those who train less, frequently indoors, and/or in a more structured fashion would tend to fall towards the upper end of this range. Unless there is a specific reason to do otherwise (e.g., transitioning from using a spreadsheet to track TSS to using the Performance Manager within WKO+), the same value should be assigned to CTL and ATL (i.e., TSB is assumed to be zero). Over time, of course, an individual’s CTL will become evident, in which can it may prove necessary, or at least desirable, to go back and revise these initial estimates. Of course, as discussed under point #2 above the calculated values for CTL, ATL, and TSB should be interpreted cautiously following such a “seeding” until sufficient data are available."
I would think you just treat the time as a break and your CTL and ATL will be low as they should be cause you have not trained. By doing this, and commencing your training program after your break, you will be able to go back and see how you perform after such a break and use that info when planning time off the bike in the future or if you MUST take time off for unforseen circumstances and have a priority race coming up then the historical data will help you plan for that. I believe that this is what the performance manager is partially designed for.
If I am wrong then please correct me as I don't have first hand experience using a PM but this is how I understand it and as well as the benefits of using the availiable software to analyse your performance and past training and can tweak it for future events. i.e you might find that after a 2 week break then 10 weeks solid training you are flying then in future you can look back and see how you slowly increased your training load to reach the level you are at.
I'm sure there are many posters here that will have a better idea than me but thats my take