Unless a specific address is specified in an interface section (which obviously would not be in the default section of the configuration file) pltime and vltime are ignored and the default values are always used.

For example the following file /etc/rtadvd.conf does nothing:

default:\
        :vltime=86400:\
        :pltime=3600: 
ix1:\
        :vltime=86400:\
        :pltime=3600:
ix1.4:\
        :vltime=86400:\
        :pltime=3600:

A trace on a SLACC-configured machine that gateway advertises routes to shows:
[root@NewFS ~]# tcpdump -v -i mce0 icmp6 and ip6[40] == 134
tcpdump: listening on mce0, link-type EN10MB (Ethernet), snapshot length 262144 bytes

18:54:16.990562 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 56) fe80::2e0:b4ff:fe68:f895 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 56
        hop limit 64, Flags [none], pref medium, router lifetime 1800s, reachable time 0ms, retrans timer 0ms
          source link-address option (1), length 8 (1): 00:e0:b4:68:f8:95
          prefix info option (3), length 32 (4): 2606:83c0:8000:1b00::/64, Flags [onlink, auto], valid time 2592000s, pref. time 604800s

The configured values are ignored.

Most of the time this is an annoyance -- if your gateway gets a new delegation rtadvd will correctly and almost-immediately deprecate the old SLACC addresses your hosts have, so the impact is that their list of "old" (depreciated) address can get quite large however operation is not impacted.  Those MOST of the time this is an annoyance in that across reboots you have one deprecated address for each boot (if your delegation floats) or re-delegation event.

There is, however, one very bad circumstance impacting connectivity that can arise -- if the gateway running rtadvd is rebooted or crashes and then fails to get a delegation on restart the old IPv6 host addresses on ALL of your SLACC-configured hosts remain with valid routes and do not expire for an extremely long period of time.  The hosts will thus try to communicate on those addresses they obtained via SLACC but they go nowhere and this has a quite-serious network performance impact in that connection attempts (if IPv4 is available) will time out trying to reach IPv6 endpoints that are not accessible.  The only way to clear them is to (1) wait for the default vltime to expire, (2) manually delete them from each interface on each SLACC-configured host or (3) reboot all the SLACC configured hosts.

Mitigating this risk to a large degree could be done by setting both pltime and vltime to a reasonable period (e.g. 30 minutes and one hour respectively) and thus after an hour with no updated transmissions the advertised routes would thus expire.  However, with rtadvd ignoring the configuration directives there's no way to do this, and since you do not know the prefix or address in question for each interface until the delegation occurs there's no real fix unless you have an exit script edit that file and "kick" rtadvd each time delegations update.

This should not be necessary; if you specify a pltime and vltime they should be honored -- and in addition the man page specifically notes a "default:" interface entry that includes both the vltime and pltime parameters, thus effectively stating that they can be set there and then overridden for specific items (e.g. dns servers and suffixes)